docs: add Attachment Model overview to enhancements reference

[dereuromark]

Summary

Adds an "Attachment Model" overview to docs/reference/enhancements.md documenting where each block construct expects its attribute block. Crosslinked from docs/guide/syntax.md.

Why

djot-php has accumulated several attribute extensions (list items, definition list elements, table rows, table cells) each with its own attachment position. The individual sections document the what; this overview documents the governing principle — attributes precede their target; adjacency selects the tighter target — and the deliberate "last-content-line" exceptions for list items and <dd>.

Also documents two things readers consistently ask about:

• The list marker glyph is not an addressable AST node — use CSS ::marker.
• start, type, and reversed are <ol>-only HTML attributes and are silently stripped from <li> / <dd> output (the strip itself landed in fix: strip <ol>-only attrs (start, type, reversed) from <li>/<dd> #187).

No behavior change

Docs-only.

Related

• fix: list-item {.x} line no longer terminates the list #186 — list-item {.x} terminator fix (merged)
• fix: strip <ol>-only attrs (start, type, reversed) from <li>/<dd> #187 — strip <ol>-only attrs from <li>/<dd> (merged)
• Track upstream jgm/djot#262 for marker-adjacent list-item attribute syntax #185 — track upstream Proposal for syntax for attributes on list items jgm/djot#262 marker-adjacent syntax

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.12%. Comparing base (7cd1cff) to head (8f58401).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #188   +/-   ##
=========================================
  Coverage     91.12%   91.12%           
  Complexity     3354     3354           
=========================================
  Files            99       99           
  Lines          8575     8575           
=========================================
  Hits           7814     7814           
  Misses          761      761           

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