Add minimal default theme to zola init

[nooscraft]

Fixes #3137

Summary

This PR adds a minimal default theme that is automatically generated when running zola init, providing new users with a working site out of the box.

Motivation

Currently, zola init creates empty directories, which results in errors when users try to run zola serve immediately. This creates a rough first experience compared to other SSGs like Jekyll, 11ty, and Hugo (with themes).

Approach

Following the maintainer's guidance to use an 11ty-style minimal approach, this implementation:

• Generates essential files automatically
• Provides ~150 lines of minimal CSS
• Uses system fonts only
• Creates a max-width container (720px) for readability
• Includes basic templates demonstrating Zola's template syntax
• Remains minimal enough that users won't feel bad replacing it

Changes

Files Generated by zola init

1. .gitignore - Ignores public/ and .DS_Store
2. README.md - Basic getting started guide with zola serve and zola build commands
3. content/_index.md - Homepage content with TOML front matter
4. templates/base.html - Base HTML5 template with semantic structure
5. templates/index.html - Homepage template extending base
6. templates/page.html - Single page template
7. templates/section.html - Section list template with page iteration
8. static/style.css - Minimal stylesheet (~150 lines)

CSS Features

• Reset and box-sizing
• System font stack
• Responsive max-width container (720px)
• Readable typography (line-height 1.6)
• Basic link styling (#0066cc)
• Simple header with navigation
• Footer with subtle border
• Code block styling
• Minimal color palette (black text, gray accents, blue links)

Testing

All existing tests pass, plus updated test assertions for the new generated files.

Manual Testing

# Create a new site
zola init mysite
cd mysite

# Build immediately (works!)
zola build

# Serve immediately (works!)
zola serve

The site now works immediately without any additional setup or configuration.

Design Philosophy

• Intentional but minimal - Looks complete, not broken, but easy to replace
• No dependencies - No JavaScript, no web fonts, no build tools
• Educational - Templates demonstrate Zola's syntax for new users
• 11ty-inspired - Follows the maintainer's preferred approach

Backward Compatibility

✅ Fully backward compatible:

• All existing tests pass
• --force flag works as expected
• Directory structure unchanged
• Only adds files, doesn't modify behavior

Screenshots

Before (Current Behavior)

$ zola init mysite
$ cd mysite
$ zola serve
Error: Failed to build the site
Reason: Failed to render index page

After (This PR)

$ zola init mysite
$ cd mysite
$ zola serve
Building site...
-> Creating 0 pages (0 orphan) and 0 sections
Web server is available at http://127.0.0.1:1111

Site displays immediately with clean, minimal styling.

Future Enhancements (Not in this PR)

Potential additions for future consideration:

• Optional --minimal flag to skip theme generation
• Sample blog post with taxonomies
• Dark mode support via CSS variables
• Additional template examples

Related Discussion

• Issue: Improving zola init - thoughts? #3137
• Research on other SSGs: See SSG_COMPARISON.md in discussion