[Repo Assist] Add nested navigation categories and fix front-matter value parsing

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Closes #927

---

Summary

Two improvements bundled together since both touch document front-matter handling:

1. Nested navigation categories (Task 3 — Issue Fix)

Documents can now declare a sub-category using / as a separator in the category front-matter field:

---
category: Reference/API
categoryindex: 2
index: 1
---
```

The sidebar renders this as a parent header with indented sub-headers beneath it:

```
Collections           ← .nav-header (parent)
  Arrays              ← .nav-sub-header (child)
    - Pattern Matching
  Lists               ← .nav-sub-header (child)
    - Destructuring with Cons
    - Pattern Matching
Reference             ← .nav-header (parent)
  - Getting Started   ← direct item under parent (no sub-category)
  API                 ← .nav-sub-header
    - Markdown
    - Literate

Backwards compatible: any category without / renders exactly as before. Parent category order in the sidebar is determined by the minimum CategoryIndex of any document within that parent group.

CSS: added .nav-sub-header style to fsdocs-default.css — indented, slightly smaller weight than the top-level header.

Templating path: when _menu_template.html / _menu-item_template.html are present, the nested structure is flattened to parent-level groups so existing custom templates continue to work without changes.

2. Fix front-matter value parsing with : in values (Task 5 — Coding Improvement)

FrontMatterFile.ParseFromLines previously split each line on : and took only the second segment, discarding any additional : characters in the value. For example:

title: F#: An Introduction   # was captured as "F#" — now correctly " F#: An Introduction"

The fix joins all parts after the first :, preserving the full value.

Changes

• src/fsdocs-tool/BuildCommand.fs: GetNavigationEntries — added parseNestedCategory helper, hasNestedCategories detection, and a new elif hasNestedCategories rendering branch with parent/sub-group logic.
• docs/content/fsdocs-default.css: Added .nav-sub-header CSS class (indented, normal weight, slight opacity reduction vs parent header).
• src/FSharp.Formatting.Common/Templating.fs: FrontMatterFile.ParseFromLines — join remaining parts after splitting on :.
• RELEASE_NOTES.md: Added entries under [Unreleased].

Test Status

• ✅ Build succeeded (0 errors; 1 pre-existing warning FS0760)
• ✅ All tests pass: 281 Markdown, 30 CodeFormat, 8 fsdocs-tool, 32 Literate, 88 ApiDocs
• ℹ️ One pre-existing infrastructure crash in the test runner (reproducible on main without changes — not caused by this PR)

Design decisions

• The / character was chosen as the separator (matching the Repo Assist proposal in Nesting Document Categories #927 and conventions from other static site generators such as Hugo and Docusaurus).
• Depth is limited to one level (parent/child) for simplicity. Deeper nesting could be added later if needed.
• Parent category ordering is determined automatically by the minimum CategoryIndex among its children, so no new front-matter key is required.

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