Replace RELATIVE_TO_PAGE with Page Resource helpers

[cnunciato]

In preparing to draft some how-to docs around blogging, I noticed we're doing some manual search-and-replace work to support post-path-relative image references. But it turns out there's support for this sort of thing built into Hugo, so this change replaces that logic with Page Resource helpers instead.

It also removes /images/pulumi.png (as it's a duplicate of /social/pulumi.png) and uses /social/pulumi.png as the default OpenGraph image where appropriate.

[pulumi]

🍹 The Update (preview) for stack pulumi/www.pulumi.com/staging was successful.

Resource Changes

    Name                                                                           Type                              Operation
~   docs/aws/sqs/index.html                                                        aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/aws/ram/index.html                            aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_aws/elasticloadbalancingv2/index.html         aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/f5bigip/sys/index.html                        aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_mysql/index.html                              aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/gcp/compute/index.html                        aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/kubernetes/scheduling/v1beta1/index.html      aws:s3/bucketObject:BucketObject  update
~   blog/program-the-cloud-with-12-pulumi-pearls/index.html                        aws:s3/bucketObject:BucketObject  update
~   docs/quickstart/azure/destroy-stack/index.html                                 aws:s3/bucketObject:BucketObject  update
~   docs/reference/vs/terraform/index.html                                         aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/pulumi/tests/runtime/index.html               aws:s3/bucketObject:BucketObject  update
~   docs/reference/cli/pulumi_stack_tag/index.html                                 aws:s3/bucketObject:BucketObject  update
~   blog/cd-made-easy-with-pulumi-and-azure-pipelines/index.html                   aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/azure/streamanalytics/index.html              aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_azure/core/index.html                         aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_azure/trafficmanager/index.html               aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_gcp/kms/index.html                            aws:s3/bucketObject:BucketObject  update
~   blog/advanced-typescript-type-ftw/index.html                                   aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_kubernetes/apiextensions/v1beta1/index.html   aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_kubernetes/yaml/index.html                    aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_openstack/firewall/index.html                 aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/aws/directconnect/index.html                  aws:s3/bucketObject:BucketObject  update
~   docs/quickstart/aws/next-steps/index.html                                      aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/kubernetes/apiregistration/index.html         aws:s3/bucketObject:BucketObject  update
~   blog/easy-serverless-apps-and-infrastructure-real-events-real-code/index.html  aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/aws/servicediscovery/index.html               aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/aws/dax/index.html                            aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/aws/rds/index.html                            aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/gcp/logging/index.html                        aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_aws/cloudtrail/index.html                     aws:s3/bucketObject:BucketObject  update
~   webinar/aws-fargate-and-pulumi/index.html                                      aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/azure/cognitive/index.html                    aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/gcp/sourcerepo/index.html                     aws:s3/bucketObject:BucketObject  update
~   docs/reference/cli/pulumi_stack_select/index.html                              aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/vsphere/config/index.html                     aws:s3/bucketObject:BucketObject  update
~   blog/managing-github-webhooks-with-pulumi/index.html                           aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/azure/logicapps/index.html                    aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/python/pulumi_azure/index.html                              aws:s3/bucketObject:BucketObject  update
~   blog/2018-year-at-a-glance/index.html                                          aws:s3/bucketObject:BucketObject  update
~   docs/reference/pkg/nodejs/pulumi/gcp/monitoring/index.html                     aws:s3/bucketObject:BucketObject  update
~   docs/tour/programs-packages/index.html                                         aws:s3/bucketObject:BucketObject  update
... and 1053 other changes

[justinvp]

LGTM. Thanks.