Decrease doc font size

[gagb]

This pull request includes a small change to the python/packages/autogen-core/docs/src/_static/custom.css file. The change introduces a new CSS variable to set the base font size for the HTML element.

• python/packages/autogen-core/docs/src/_static/custom.css: Added --pst-font-size-base CSS variable to set the base font size for the HTML element.

Old (landing page requires scrolling)

New (landing page fits and looks spacier; makes it easier to read the API reference as well)