docs: Add validation Github Action for docs PRs

[grafakus]

✨ Description

Related issue(s): resolves #62

This PR:

• adds a GitHub action to build the docs when documentation files are modified in a PR,
• prevents frontend build and tests to be executed when only docs files have been modified in a PR
• adds an npm script to help contributing to the documentation.
• formats all Markdown docs with Prettier (the ones that were not formatted yet),

📖 Summary of the changes

See diff tab

🧪 How to test?

• The build with the new Github Action should pass

[github-actions]

Unit test coverage

Lines	Statements	Branches	Functions
	13.05% (468/3584)	10.43% (138/1322)	9.9% (107/1080)

[simonswine]

LGTM

Not fully sure about the implications for the docs in docs/sources and other validations/builds docs/Makefile`. Are we even building the docs as part of a PR?

But gnerally agree with enforcing this

[knylander-grafana]

LGTM

Not fully sure about the implications for the docs in docs/sources and other validations/builds docs/Makefile`. Are we even building the docs as part of a PR?

We should build docs as part of the PR check if the PR has doc changes like we do in the Pyroscope repo. Our build process doesn't use any docs outside of docs/sources, so the linter should only run on those items. If you have specific questions, I can ask Jack Baldry and find out for you.

But generally agree with enforcing this

For reference, doc linter info: (https://grafana.com/docs/writers-toolkit/review/lint-prose/).

[jdbaldry]

Nice, most looks good to me

[knylander-grafana]

Since Jack has already reviewed, I'll defer to his comments.

[jdbaldry]

Docs related CI and DX look good to me

[grafakus]

@jdbaldry I've fixed some conflicts, may I ask you a final review? 🙏🏾 Thank you!

[jdbaldry]

LGTM!