## Problem
The vimdoc had several issues: integration sections (fugitive, neogit,
gitsigns) were scattered as top-level entries with no grouping, the
intro implied diffs.nvim works automatically with fugitive/neogit out of
the box, the neogit section described a highlight override workaround
that was already removed, and `extra_filetypes` didn't mention picker
support.
## Solution
- Rewrite intro to document default behavior: `gitcommit` highlighting,
conflict detection, `&diff` winhighlight — everything else is opt-in
- Remove vim-fugitive as a dependency in setup
- Add `|diffs-integrations|` parent section grouping fugitive, neogit,
and gitsigns
- Add fugitive intro paragraph mentioning `:Gdiff`
- Trim neogit section (remove stale highlight override docs)
- Trim gitsigns section (remove implementation details)
- Add `committia` config field docs
- Expand `extra_filetypes` docs to mention telescope, snacks, and
fzf-lua
d06144450c