Call for support: devhelp v2

[Komiya Takeshi]

Hi all,

Now Sphinx provides devhelp builder. It can generate devhelp v1
format. But we received a report that the v1 format was marked as
deprecated now.
https://github.com/sphinx-doc/sphinx/issues/4890

So we have to improve the builder to generate v2 format.
But I don't know it at all.
If you have time for it, please send a pull request for us.
Any information about v2 are also helpful.

Thanks,
Takeshi KOMIYA