Add tippy_glossary_base_url to fix broken glossary term links in tooltips

[svekars]

RE: #32

Problem

When using sphinx-tippy for glossary tooltips, any :term: references within a glossary term definition create broken links. This happens because the tooltip content uses relative URLs (e.g., #term-Custom-Operation) which resolve relative to the current page instead of the glossary page.

Example:

.. glossary::

   Custom Operation
      An Operation defined by users, usually a :term:`Compound Operation`.

When hovering over "Custom Operation" on test-page.html, clicking "Compound Operation" in the tooltip navigates to test-page.html#term-Compound-Operation (broken) instead of glossary.html#term-Compound-Operation (correct).

Solution

This PR adds a new configuration option tippy_glossary_base_url that allows users to specify the base URL for glossary term links within tooltips.

Usage:

# conf.py
tippy_glossary_base_url = "glossary.html"

When configured, all #term-* anchor links inside tooltip content are rewritten to include the glossary base URL (e.g., #term-Something → glossary.html#term-Something).

Testing

• All existing tests pass
• Added 5 new unit tests covering:
  • Empty base URL (no rewriting)
  • Base URL with glossary term links (correct rewriting)
  • Preservation of non-glossary links
  • Multiple glossary terms in same content
  • Configuration value propagation
• Manually tested with a real documentation set containing cross-referenced glossary terms