✨feat(config): add dynamic stylesheet loading via config.toml and page Metadata

[welpo]

Description

This PR addresses issue #117. It adds a new feature that allows users to specify additional stylesheets in the config.toml and individual page metadata. This functionality adds flexibility for custom themes and individual post styling.

PR #105 removed a simpler (and undocumented) implementation of this feature.

Changes

• Implemented dynamic stylesheet loading in the header.html template.
• Added stylesheets field to config.toml and theme.toml.
• Updated documentation in the configuration files to guide users on using the new feature.

How to use:

1. Specify additional stylesheets in config.toml under stylesheets as an array. For a single file there's no need for an array.

   [extra]
   stylesheets = ["extra1.css", "path/extra2.css"]

2. For individual pages, add stylesheets to the [extra] section in the front matter.

   +++
   title = "My Page"
   [extra]
   stylesheets = "individual.css"  # Use an array for more than one file.
   +++

Note: Since Zola processes .sass and .scss files in the sass folder into .css files in the public folder (see Zola docs), you should specify the processed .css filenames in the stylesheets field.

For example, if you have a .scss file located at sass/extra.scss, Zola will render it into public/extra.css. You should then specify extra.css in the stylesheets field like so:

[extra]
stylesheets = ["extra.css"]