docs: Add tooling to check and validate build:

[kirkrodrigues]

• Add script to check for invalid links
• Add workflow to validate build
• Fail on warnings
• Fix invalid link in log-viewer-webui README

Description

When Sphinx generates an html file from a Markdown file, it will automatically translate relative links with the .md suffix to use the .html suffix. However, it doesn't/can't do this for:

• sphinx cards;
• READMEs that are outside the docs site (in other words, outside the docs directory);
• absolute links that include the docs.yscope.com prefix.

This PR:

• Adds a script to check for invalid links, namely:
  • Links to https://docs.yscope.com that include the .md suffix; if at all, they should include .html as the suffix, or better yet, no suffix at all.
  • :link: attributes in sphinx cards that include the .md suffix; again, they should include no suffix at all.
  • This script is added as a step in the build since invalid links will lead to an invalid build.
  • This script also generates errors using GH Action's workflow command syntax.
• Fixes the invalid link in components/log-viewer-webui/README.md
• Updates the sphinx build command to fail on warnings.
• Adds a workflow to validate building the docs on both Ubuntu and macOS.

Validation performed

• Validated that https://docs.yscope.com links that include the .md suffix are detected and fail the build.
• Validated that http://docs.yscope.com links that include the .md suffix are detected and fail the build.
• Validated that sphinx card :link: attributes that include the .md suffix are detected and fail the build.
• Validated that warnings fail the build.
• Validated that invalid link errors show up as annotations on GH.
  • Note that annotations only work for files changed in the most recent comment.

[Henry8192]

A trivial change for regex, other changes LGTM :)