DOC: Update install docs for ttconv removal and Windows config dir - #32482

PR summary

This PR fixes three outdated information in doc/install/:

• (i) Removes ttconv from the bundled library list. It was removed in Matplotlib 3.10.
• (ii) Updates the environment variable and troubleshooting pages with the Windows config and cache paths. Since 3.11, both default to %LOCALAPPDATA%\matplotlib. If %USERPROFILE%\.matplotlib already exists, that directory is used instead. Updates the Windows examples to match.
• (iii) Clarifies that Linux and FreeBSD use separate .config and .cache directories by default, while macOS uses ~/.matplotlib for both.

AI Disclosure

Claude Code (Claude Sonnet 5.5 (high) ) found these during docs audit.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• [N/A] New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New features and API changes have release notes
• Documentation complies with general and docstring guidelines