Post-migration Devguide reorganization

I think the Devguide is due for some reorganization after the migration.

In particular:

• The version control setup section should link to a separate "git" page (either the pullrequest or the committing page -- see below). (see #265)
• The compile and build section should be reorganized. (see #412)
• The lifecycle of a PR page seems to expand on the quick start, but doesn't include detailed setup instructions and git commands. This page seem to be aimed at contributors (see #265). Also the Generation section should be under Preparation, alongside another subsection about running tests.
• The runtests page expands on a paragraph pullrequest/preparation.
• The silence warnings and fixing easy issues are both 2-paragraphs pages that might be moved as sections of other pages.
• The tracker and triaging page should be updated and perhaps merged (see #308 and #339).
• The where to get help and following Python's development could be merged.
• The committing page seems to be aimed to core-dev, but since now the workflow for both core devs and contributors is quite similar, it should perhaps be merged with the pullrequest page. Parts of this page should be moved to other pages, and only the content relevant to core devs should be left there. (see #262 and #263)
• The continuous integration page should be updated. (see #290)
• The experts page should be moved closer to the developers page. (see #901)
• The mercurial for git developers page is now unnecessary, and it should either be removed, or better reversed and become "git for mercurial developers". (see #258)
• The adding to the stdlib page should first list what are good and bad addictions (with examples) and then explain the process to add something (see #35).

The other pages don't have any major issue :)

Review the unchecked Devguide pages listed in the issue, especially runtests, silence warnings, fixing easy issues, help and communication channels, and adding to the stdlib. First confirm which completed items and referenced issues still reflect the current site, then determine the remaining page moves and content updates. Done means an agreed reorganization is implemented and the affected links and navigation remain coherent.