Add docs build workflow.

[ederjc]

Description of Change

This adds a reusable workflow for build checking of docs for repos using Sphinx and this folder structure:

docs/
  requirements.txt
  Makefile
  ...

and can be called from any repository using (for example):

 name: Build docs
 on:
  ...
 jobs:
  call-workflow:
    uses: Infineon/makers-devops/.github/workflows/docs_build.yml@main

Workflow Test

Tested here: Infineon/arduino-core-psoc6#81

Checklist

Remove items that do not apply. For completed items, change [ ] to [x].

• PR description included and stakeholders cc'd
• All tests pass
• Tests are changed or added
• Relevant documentation, tutorials, templates and examples are changed or added
• PR release notes describe the change in a way relevant to developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Please add a concise description for developers to read in the release notes, or 'none' if notes are not relevant to developers.

[jaenrig-ifx]

Nice :D