Move npm-check commands from style-guide.md to pr-checks.md

[vitorvasc]

Description

Closes #6601

As raised in #6601, it's not intuitive to have the npm checks commands listed on the Style guide page. We should probably move them to Pull request checks page or create a new one.

By taking a closer look at both pages, most of the commands described on the style-guide.md page are also on pr-checks.md:

Command in style-guide.md	Present in pr-checks.md
npm run fix:all	✅
npm run check:text	✅
npm run check:text -- --fix	✅
npm run check:markdown	✅
npm run fix:markdown	❌
npm run check:spelling	❌
npm run fix:dict	✅
npm run fix:format	✅
npm run fix:filenames	✅

Proposal

style-guide.md

1. Remove the redundant commands from the Style guide page.
2. Add a new section named Fixing validation issues, leading to the Pull request checks page.

pr-checks.md

Include the previously missing commands (npm run fix:markdown and npm run check:spelling).

---

The only command left in style-guide.md, at least for now, is npm run fix:format in the File format section. Does it make sense to keep it where we mention Prettier?

[tiffany76]

Thanks, @vitorvasc! I think @chalin will want to take a look at this PR when he's back online (mid-July), if you're okay waiting.