Add automatic layout detection and title generation

[ChristopherA]

This PR adds automatic layout detection and title generation as core features of BaseTheme, reducing boilerplate in content files while maintaining flexibility.

Changes

1. Added core plugins:

   • jekyll-titles-from-headings for automatic page title generation
   • jekyll-default-layout for automatic layout detection

2. Added plugin configuration:

   titles_from_headings:
     strip_title: true      # Removes the first H1 from content
     strip_spans: true      # Removes span tags from titles
     collections: true      # Enables for collections

3. Added _layouts/home.html template for index pages

4. Updated README.md with new features and updated examples

5. Updated documentation of layout hierarchy and front matter requirements

Automatic Layout Detection

Files will now automatically get the correct layout based on their location:

• index.md → home layout
• _posts/*.md → post layout
• Other .md files → page layout
• .html files → default layout

Automatic Title Generation

Page titles are now automatically generated from the first H1 heading in the content, removing the need to specify titles in front matter.

Breaking Changes

Users should update their _config.yml to include the new required plugins.