Update Contribution guidelines and PR template

[waldyrious]

This commit updates the commit message guidelines in CONTRIBUTING.md:

• Add example commit messages for translations and for changes that affect multiple pages;
• Prefix each example with a description of the type of change it refers to.

It also updates the PR template, to make it more compact and reduce duplication of information included in CONTRIBUTING.md:

• Remove horizontal rule at the top
  (the comments are sufficient as visual separation);
• Shorten the comments to make them more compact and more likely to be read;
• Make each checklist entry take a single line;
• Remove blank lines between checklist entries.

This is a follow-up to the discussion at #2512.

[sbrl]

Looks great, @waldyrious! Much briefer than our existing template.

[waldyrious]

@sbrl and @pxgamer, I've updated the PR to fix the relative URL to CONTRIBUTING.md so that it works from the PR page, as discussed in the inline comments above.

[waldyrious]

Please don't merge yet, I need to fix the issue @pxgamer pointed out in the review comments.

[waldyrious]

Update: I've confirmed the current URL format works as intended (see the inline conversation above for details). Feel free to merge this.

[waldyrious]

Oh no, @pxgamer, you squash-merged my carefully crafted separate commits 😢

Not a big deal of course :) but in the future, please keep in mind the second-to-last bullet point here when merging PRs.

[owenvoke]

Ah, damn it. I apologise, I completely forgot about commit history.

[waldyrious]

No problem! 😃 this sort of thing is better gradually acquired as part of the development culture/process, rather than by memorizing rules exhaustively.