ci: add markdownlint

[rwaffen]

No description provided.

[jay7x]

TY! I'll adjust it and merge after rubocop PR!

[jay7x]

This works fine, but now I think we may want to use Ruby-based markdown linter instead:

• Linter itself https://github.com/markdownlint/markdownlint
• GHA: https://github.com/actionshub/markdownlint

Will compare both and see..

[tuxmea]

@jay7x Any news regarding your tests with ruby based markdownlint?

[jay7x]

Forgot about this tbh.. will try to have a look tonight

[jay7x]

Well.. I spent some time on this. Below are quick results.

➜  openvox-docs git:(master) ✗ grep -c 'MD[0-9]*' mdl.output          
88107
➜  openvox-docs git:(master) ✗ grep -c 'MD[0-9]*' markdownlint-cli2.output 
88419
➜  openvox-docs git:(master) ✗ tail -n1 mdl.output 
9.65s user 0.60s system 99% cpu 10.258 total
➜  openvox-docs git:(master) ✗ tail -n1 markdownlint-cli2.output 
11.23s user 0.76s system 132% cpu 9.022 total

NOTES:

• mdl.output is Ruby-based linter output (time bundle exec mdl **/*.md **/*.markdown > mdl.output 2>&1)
• markdownlint-cli2.output is Node-based markdownlint-cli2 output (time npx markdownlint-cli2 **/*.md **/*.markdown > markdownlint-cli2.output 2>&1)
• Both outputs were edited to remove non-rule output
• Run was made on a non-clean repo, so there is a mix of old and new repos actually.. I don't think that really matters in this case though.

[jay7x]

Speed-wise I'd say they are quite similar.

Strong sides of linters:

• markdownlint-cli2 has slightly more rules (looking at the output).
• mdl is Ruby-based, so we can bake it into our future SDK.