docs(conf): Use sphinx_autodoc_fastmcp and remove local _ext

why: Drop the inlined fastmcp_autodoc extension in favor of the gp-sphinx
package; configure tool modules, area map, and section badges.
what:
- extra_extensions and fastmcp_* settings in docs/conf.py
- remove docs/_ext/fastmcp_autodoc.py; drop _ext from sys.path

Author: tony