docs: revert to plain mdBook styling (#414)

Remove visual enhancements added in previous commit:
- Remove mdbook-admonish callout boxes
- Remove custom CSS theme
- Simplify introduction and quickstart content
- Keep plain mdBook defaults

This branch preserves the plain mdBook approach for comparison.
Main branch has the enhanced styling if we want to use it later.

Author: joshrotenberg

docs/book.toml

@@ -10,16 +10,11 @@ preferred-dark-theme = "ayu"
 git-repository-url = "https://github.com/joshrotenberg/redisctl"
 git-repository-icon = "fa-github"
 edit-url-template = "https://github.com/joshrotenberg/redisctl/edit/main/docs/{path}"
-additional-css = ["theme/custom.css", "./mdbook-admonish.css"]
 
 [output.html.fold]
 enable = true
 level = 1
 
-[preprocessor.admonish]
-command = "mdbook-admonish"
-assets_version = "3.0.2" # do not edit: managed by `mdbook-admonish install`
-
 # Temporarily disabled for reorganization
 # [preprocessor.lint]
 # command = "mdbook-lint"