docs: Add ThemeConfig section to User Guide

[dangotbanned]

Description

Work-in-progress

What

Show how this benefits:

Typing users

• Autocomplete
• Static validation
• Doesn't require using class-syntax
  • Can be used to see docs
• Compare the UX to that of Vega Editor

Non-typing users

• Class-syntax is required for completions
• Still likely an improvement over using a dict
• Point to resources on how they can get the full benefits
  • ipython/jupyter
    • Language servers/plugins/extensions
  • IDE
    • typing-typing-related-tools
    • I'd probably call out VSCode w/ pylance as a common case

Where

Either

• Following defining-a-custom-theme
• or as a subsection of it

Related

• feat: Adds ThemeConfig (TypedDict) #3536
• refactor: alt.themes -> alt.theme #3618 (comment)
• feat: Adds ThemeConfig (TypedDict) #3536 (comment)

Have you considered any alternative solutions?

We can leave chart-themes in the same state as left by #3618

I think @joelostblom's comment shows why this feature warrants further explanation.

The only alternative I see would be linking to typing-spec-TypedDict.
However, I did this in #3536-description and that alone did not suffciently convey the benefits to reviewers