Enhance Sitemap Generation: Add lastmod based on Git History

Adding <lastmod> tags to the sitemap entries is a crucial enhancement for SEO. It provides search engines with information about when a page was last modified, which can improve crawling efficiency and ensure that fresh content is indexed more quickly.

This ticket proposes to enhance the buildScripts/generate-seo-files.mjs script to include the <lastmod> tag for each URL entry in the generated sitemap.xml.

Implementation Details:

• The generate-seo-files.mjs script will need to be modified to fetch the last modification date for each content file (e.g., Markdown files in the learn directory).
• A robust way to determine the last modification date is to query the Git history for the last commit date of each file.
• The fetched date should then be formatted according to the W3C Datetime format (e.g., YYYY-MM-DD) and included within a <lastmod> tag for each <url> entry in the sitemap.xml.

Acceptance Criteria:

• The sitemap.xml generated by generate-seo-files.mjs includes a <lastmod> tag for each <url> entry.
• The value of the <lastmod> tag accurately reflects the last modification date of the corresponding content file, as determined by its Git history.
• The date format adheres to the W3C Datetime format.