Utilize sitemap for crawling Docs website pages

[Burhan-Q]

Related to #46

Implements sitemap crawling for the Ultralytics Documentation Bot to dynamically fetch and filter pages from the Docs website.

• Adds a new function fetch_sitemap to retrieve and parse the sitemap XML from the Docs website, filtering URLs to include only desired pages.
• Modifies docs_choices to utilize the sitemap for obtaining URLs, generating Discord Embeds for each URL with a title, author, and footer.
• Implements a check to ensure titles for generated Discord Embeds are less than 25 characters.
• Adds a function validate_sitemap_urls to verify the validity of URLs obtained from the sitemap, ensuring they are reachable and return a 200 status code.

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