✨ feat(header): add support for canonical URLs

[welpo]

Description

This PR enhances tabi's SEO capabilities by introducing a flexible system for setting canonical URLs.

The feature allows for setting canonical URLs at individual page or section levels, with a fallback to a dynamically generated global base_canonical_url, if specified.

Resolves #151.

Changes

• Added canonical_url as an optional front matter attribute in both pages and sections under the [extra] section.
• Introduced a dynamic fallback mechanism that crafts the canonical URL by replacing the $base_url in the current URL with the base_canonical_url specified in config.toml.
• Updated HTML templates to conditionally include <link rel="canonical"> and <meta property="og:url"> based on the newly introduced logic.

How to Test

1. Add canonical_url to the [extra] section of a page or section's front matter.

   [extra]
   canonical_url = "https://example.com/custom-canonical-url"

2. Alternatively, set a dynamic global base_canonical_url in config.toml.

   base_canonical_url = "https://example.com"

3. Build the site and verify that the canonical URLs in the HTML headers are set as expected.

Additional Information

This feature is beneficial for those running multiple domains with the same content. By setting base_canonical_url dynamically, it will automatically adapt the canonical URLs based on the current URL's $base_url. For example, for a page with URL $base_url/blog/post1, and a base_canonical_url set to "https://example.com", the generated canonical URL will be "https://example.com/blog/post1".