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
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