markup/tableofcontents: Add pre/post config options

[nathannewyen]

Add Pre and Post configuration options to customize the HTML wrapper around the table of contents.

This allows users to add custom elements like headings for accessibility:

[markup.tableOfContents]
  pre = '<nav id="TableOfContents"><h2>Contents</h2>'
  post = '</nav>'

The pre/post parameters are optional in ToHTML() for backward compatibility - existing templates continue to work without changes.

Fixes #8338

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[github-actions]

AI Detection Analysis 🔍

Confidence Score: 25%

Reasoning: The pull request adds new configuration options (pre/post HTML wrappers) to an open-source project, with carefully explained rationale and code changes. The tone is pragmatic, technically accurate, and formatted in a way that suggests knowledge of both the underlying system and the expectations of other developers. There is a clear reference to issue #8338, and the description integrates user-oriented benefits (such as accessibility enhancements) with implementation details, balancing clarity and precision—traits more typical of human authorship.

Key Indicators:

• Contextual and user-focused rationale: mentions accessibility and backward compatibility.
• Use of idiomatic language and concise phrasing typical of open-source collaboration.
• Manual TOML example included, demonstrating real-world use.
• Specific and in-place code modifications that reflect nuanced understanding of the codebase.
• Fixes a known issue (Add pre/post config to ToC #8338), demonstrating integration into the project’s maintenance workflow.

These elements suggest a human developer familiar with both the technical domain and the collaborative practices of open-source development.

✅ No strong indicators of AI generation detected

[jmooring]

Let's put this on hold per my comments here:
#8338 (comment)