docs: move vimdoc to `doc/`

First off, thanks for writing a vimdoc :) . Apologies for not creating an issue to consult first. Thought a PR would be faster.

Problem: :helptags don't seem to be read when they are generated at the root of the project.
Solution: :help 'runtimepath' suggests that Neovim looks in the doc/ directory for documentation tags. Moving the vimdoc file to doc/ and generating :helptags seems to work. Tested with :help nvim-treesitter-context.

nvim-treesitter / nvim-treesitter-context