Add inbound links validation

[Mpdreamz]

Adds two commands to docs-assembler

$ docs-assembler link validate-inbound-all

and

$ docs-assembler link validate-inbound-local

validate-inbound-all

Validates all repositories published to the link index. Each links.json has cross_links section declaring all cross links used in that docset. This command will validate each cross link actually still resolves.

validate-inbound-local

Similarly this validates all published cross_links still resolve using a local links.json file to override the definition for a given repository.

This is exposed as a github action:

    - name: Validate Inbound Links
      uses: elastic/docs-builder/actions/validate-inbound-local@main

Which may be called after a documentation build. Will hook this up to be part of the default publish flow in a follow up PR.

This will only validate crosslinks to the current repository. So a failure in one repository shouldn't fail all repositories.

[reakaleek]

🚀 as always