feat: add fileAppender parameter to GenMarkdownTreeCustom

[algojogacor]

This is an independent contribution submitted in a personal capacity and is not associated with any competition or organized event.

Summary

Add a fileAppender parameter to GenMarkdownTreeCustom, enabling callers to append content (such as footers or custom metadata) to generated markdown files — mirroring the existing filePrepender behavior.

Root Cause

GenMarkdownTreeCustom accepts a filePrepender func(string) string that writes content before the generated markdown, but there is no equivalent mechanism to append content after the markdown body. Users who need to inject footers must resort to post-processing the generated files.

Closes: #1140

Fix

Added a fileAppender func(string) string parameter to GenMarkdownTreeCustom. The appender is called with the filename after GenMarkdownCustom writes the markdown content, following the same pattern as filePrepender.

• GenMarkdownTree passes emptyStr for the new parameter (backward compatible)
• Recursive calls pass fileAppender through
• Doc comment updated

Testing

• go build ./doc/... compiles without errors
• Existing GenMarkdownTree behavior is unchanged — the default emptyStr appender writes nothing

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