docs: update and provide correct information

[gildesmarais]

This pull request makes significant improvements to the documentation and user experience for the html2rss project. The main focus is on clarifying instructions, making guides more beginner-friendly, updating navigation and terminology, and enhancing error handling. The changes span the main landing page, guides for both the web application and Ruby gem, configuration documentation, and the 404 error page.

Documentation and Getting Started Improvements

• Updated index.md and web-application/getting-started.md to provide clearer explanations of RSS, html2rss, and getting started options for both beginners and advanced users. The landing page now better distinguishes between html2rss-web and the Ruby gem, and emphasizes the Feed Directory for new users. [1] [2]
• Rewrote html2rss-configs/index.md to use simpler, friendlier language and a step-by-step approach for creating custom RSS feeds, including improved explanations of YAML configuration and selectors. [1] [2] [3] [4] [5] [6] [7]

Navigation and Link Updates

• Updated internal links to use the new /get-involved/contributing path instead of /contributing for consistency and improved navigation. [1] [2]

Feature Documentation and Reference Enhancements

• Improved documentation for automatic feed generation in web-application/how-to/use-automatic-feed-generation.md, including step-by-step instructions for enabling and using the feature.
• Updated Ruby gem installation instructions to clarify the required Ruby version (now 3.2 or higher) and replaced Docker setup instructions with GitHub Codespaces for easier cloud development. [1] [2]
• Added and clarified selector and extractor options in ruby-gem/reference/selectors.md, including new columns and examples for attributes and static values, and clarified enclosure usage. [1] [2] [3]
• Fixed typo in stylesheet reference (.xls → .xsl) in ruby-gem/reference/stylesheets.md.
• Added missing selector example for JSON scraping in ruby-gem/how-to/scraping-json.md.

Error Handling and User Guidance

• Enhanced the 404.html error page with a more helpful message and links to key resources, improving user experience when encountering broken links.