Improve site structure and fix various issues

[f-allian]

This pull request introduces several improvements to accessibility, SEO, plugin management, and external service integrations in the site configuration and templates. The most notable changes include improved accessibility attributes, updated Open Graph and sitemap settings for better SEO and social sharing, and cleanup of unused plugins and dependencies.

Accessibility improvements:

• Added aria-label and aria-hidden attributes to navigation and theme toggle icons in _includes/header.html to enhance accessibility for screen readers. [1] [2]

SEO and social sharing enhancements:

• Enabled Open Graph meta tags and set a default Open Graph image in _config.yml for richer social media link previews.
• Updated the sitemap URL in robots.txt to use the www. subdomain, ensuring consistency and proper indexing by search engines.

Plugin and dependency management:

• Removed the unused jekyll-get-json plugin from both _config.yml and Gemfile to streamline the build process. [1] [2]
• Updated the Mermaid CLI install command in .github/workflows/deploy.yml to use the correct package name (@mermaid-js/mermaid-cli).

Code and content quality:

• Improved alternative text for the profile image in _layouts/about.html for better accessibility and SEO.
• Enhanced logging and added a timeout for fetching external posts in _plugins/external-posts.rb to improve reliability and debugging.

Other minor improvements include fixing the handling of the Giscus theme iframe [1] cleaning up meta tags and layout structure in _layouts/default.html [2] [3] and clarifying the site description in _config.yml [4].