Link checking for links relative to elastic.co

[bmorelli25]

The following links fail link checking:

:: fail: Documentation.Builder.Log[0] `/downloads/elasticsearch` does not exist. resolved to `/Users/brandon/Documents/GitHub/elasticsearch.md/docs/downloads/elasticsearch (/Users/brandon/Documents/GitHub/elasticsearch.md/docs/rpm.md:12)
:: fail: Documentation.Builder.Log[0] `/downloads/past-releases` does not exist. resolved to `/Users/brandon/Documents/GitHub/elasticsearch.md/docs/downloads/past-releases (/Users/brandon/Documents/GitHub/elasticsearch.md/docs/rpm.md:12)
etc.

The links are meant to resolve to the following, respectively:

https://www.elastic.co/downloads/elasticsearch
https://www.elastic.co/downloads/past-releases

Syntax:

[Download {{es}}](/downloads/elasticsearch)
The latest stable version of Elasticsearch can be found on the [Download Elasticsearch](/downloads/elasticsearch) page. Other versions can be found on the [Past Releases page](/downloads/past-releases).

[bmorelli25]

From @siamakp-elastic

Maybe we can add a special syntax to mark some links as external so that they are not flagged by docs-builder. Something like:
[Support Matrix](//support/matrix)
or:
[Support Matrix](!/support/matrix)

Basically prefix the link with ! or an extra /.

[bmorelli25]

Related link checking issues:

• Make anchor link checking case-insensitive #288
• Link checker fails on anchor links within other elements #285

[reakaleek]

Suggestion:

I think we should use the Absolute URL and migrate these links to https://elastic.co/support/matrix.

If we ever decide to use the docs.elastic.co subdomain this might break.

Also it's more explicit.

[bmorelli25]

Seems easy enough. It look like this can be fixed by regex-ing for \]\(\/ and replacing with ](https://elastic.co/