docs: generate vim help pages in norg from wiki markdown

[champignoom]

• Add :help neorg-wiki, generated from wiki with https://github.com/pysan3/minorg and several pandoc filters
• Rename :help neorg into :help norg
• Rename :help neorg-cheatsheet into :help norg-cheatsheet
• Remove :help neorg-breaking-changes, which didn't exist anyway
• Remove all other :help neorg-*, which are more noisy than helpful

One can now browse neorg help pages with neorg's own infrastructure, like hopping among pages, and gO for showing a table of content for the options.

TODOs

(for the wiki itself)

• Add missing links to entries (like Default-Keybinds.norg) into Home page body

[champignoom]

can you add a CI pipeline to generate these automatically on merge with main?

I don't really know how, but here's the generator https://github.com/champignoom/neorg-help-gen.