Add font and window geometry settings to "Quick Start" example config

[ernstki]

No matter what search terms I tried, for the life of me I couldn't find the config setting for initial window geometry using the docs site's built-in search.

Really the config settings to set the initial window geometry are quite straightforward, they're just not surfaced in the docs (IMO) in a place where a new user would expect to see them.

What this PR does is put those config settings front and center, right in the "Quick Start" section.

As an entertaining aside, I tried asking a chatbot LLM and it hallucinated some very-plausible-but-nonexistent wezterm config settings. So eventually I turned to the issue tracker, discovered #256 with a search, where others mentioned similiar problems finding these settings in the docs.

[ernstki]

@wez If you're amenable to it, I'd also like to add sections for window geometry and font size to the "Colors & Appearance" page. I can push more commits to this PR to simplify things, if that's acceptable.