Add C-h b do call describe-bindings and C-h k to call describe-key

[svetlyak40wt]

I know there is a C-x ? bound to describe-key, but mappings mentioned in the title are default in Gnu Emacs, and I think those who come from it will be happy to find these ways to learn about Lem's commands.

It would be nice to have describe-mode bound to C-h m as well, but I don't see such interactive command now.

[snmsts]

It might come from my custom of using C-h as delete-previous-char... I would prefer keys which start multi stroke command should be lessen than Emacs default. How about sacrifice C-x ? instead of C-h so C-x ? b for bindings C-x ? k for key. m for mode.

[svetlyak40wt]

This will be ok if we let people know about these important commands with making an onboarding by providing a tips:

https://github.com/cxxxr/lem/issues/410

[snmsts]

with approval from @cxxxr I'll do...

[svetlyak40wt]

Let me make this change in my pull:

https://github.com/cxxxr/lem/pull/408