help: add topic about evolution, based on text from evolve extension
I've taken the text produced by hg help evolution when the evolve
extension is enabled and made that available by the same command with
just hg core. Changes I've made:
- Added "(EXPERIMENTAL)" to the title. (That doesn't hide the topic from hg help, though.)
- Replaced old-style experimental.evolution=<names> config by new-style experimental.evolution.<name>=true.
- Replaces a "obsolete markers" by "obsolescence markers".
- Removed most content from "Current feature status".
When the evolve extension is enabled, its help text takes precedence.
Differential Revision: https://phab.mercurial-scm.org/D10516