Description

• The current user guide lacks links to relevant information.
• The only way to know what's new in the new version is to look at Changelog, but it doesn't show specific usage.
• This PR shows you how to add links to related information.

See https://forum.codeigniter.com/thread-80701-post-392042.html#pid392042

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide