## Problem
Integration docs (fugitive, neogit, gitsigns) were scattered as
top-level sections with no grouping, making it hard to find all
supported plugin integrations in one place.
## Solution
Add `|diffs-integrations|` parent section that groups all integration
subsections under a single TOC entry. Fugitive, neogit, and gitsigns are
now subsections (using `---` separators) under the new `INTEGRATIONS`
heading. The intro paragraph documents the two attachment patterns:
automatic (config toggles like `fugitive = true`) and opt-in
(`extra_filetypes`). Conflict resolution and merge diff resolution
remain as standalone top-level sections. TOC renumbered accordingly.
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