Add GH action for posting docs preview

[nkammah]

Should be merged after https://github.com/elastic/docs/pull/2843/files and it looks as below (when docs PR are opened):

[pquentin]

Thanks! LGTM.

[pquentin]

@nkammah Thanks! What is the command to trigger Buildkite builds for community contributions?

For Buildkite integration tests I've configured the pr-bot in https://github.com/elastic/elasticsearch-py/blob/8.11/.buildkite/pull-requests.json as documented in https://docs.elastic.dev/ci/configuring-pull-requests-for-buildkite, but commenting buildkite test this please did not trigger any Buildkite docs in #2252 (It does trigger a Jenkins build however: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch-py+pull-request+build-docs/948/)

[nkammah]

Ha good point @pquentin - I forgot to document it; the below commands are supported:

run docs-build
run docs-build rebuild
run docs-build warnlinkcheck
run docs-build skiplinkcheck (takes precedence over the warnlinkcheck option)

[pquentin]

Thanks! I'll try that next.

(It would be nice if buildkite test this please or an equivalent universal command could build the docs too, as is done today, which would avoid learning new commands.)

[nkammah]

Ha - that can be arranged, by maybe leveraging the always_trigger_comment_regex option of the bot and support some parameters. Could you kindly open an issue here?

[pquentin]

Looks like this is now supported, thank you!