ci: upstream docs conformance validation

[crazy-max]

needs docker/docs#14905 or docker/docs#15194

adds a job to check if links are still valid with upstream docs repository.

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[tonistiigi]

Does this tool actually pull down all the URLs (comments could help).

If it does could we filter this to only run on master or if *.md changes etc.

[crazy-max]

Does this tool actually pull down all the URLs (comments could help).

Yes it will run the validation for the whole docs repo as cross links could be broken outside our scope so we can take that into account and react accordingly (here or on docs repo).

If it does could we filter this to only run on master or if *.md changes etc.

Done

---

Also added reference yaml docs that was not covered in the validation and we have a dead link: https://github.com/docker/buildx/runs/7622740288?check_suite_focus=true#step:8:667

#23 166.5 - ./_site/engine/reference/commandline/buildx_build/index.html
#23 166.5   *  internally linking to /build/guides/color-output/, which does not exist (line 1040)
#23 166.5      <a href="/build/guides/color-output/">Color output controls guide</a>

Fixed with last commit.