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.

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.

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.

[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.

Wezterm's support for all the multitudinous color schemes and other rendering options is impressive! On a more pragmatic level, though, initial window size and font size are probably more useful to new users, and as of right now, they don't appear to have a place of their own in the docs.