Simplify How to Contribute on wiki

These steps were identified by @axelsimon and @lkatalin as ways to make the organization of How to Contribute simpler.

[x] Rename the PR page "How to contribute: Pull Request process" for consistency across pages
[x] Move the PR-related info from the How to Contribute page onto the PR page, so that it is all in one place
[x] Make sure the PR page is linked appropriately from the How to Contribute page and in the main menu
[x] Make sure there is a link from the PR page back to the DCO section on the How to Contribute page
[x] A better order under Process on the How to Contribute outline might be:
1. DCO
2. Lab machines
3. Git env set up
4. Code style / working on code / "first steps" Rust intro
5. PR process

Edit:

[x] Update "good first issue" link to reflect issues from all our repos. Depends on: https://github.com/enarx/bot/issues/29
[x] Add links to the PR process page on the "How to contribute: code" page whenever PR is mentioned
[x] Mention that a PR should link an issue
[ ] De-duplicate instructions like "git env setup" between Code, RFC pages, etc.
[ ] Mention sign-off on PR page

enarx / outreach