docs: Migrate docs about merging attribute sources from Confluence

[jonathanmtran]

The instructions for User Attributes [1] is missing the bit about configuring a CachingPersonAttributeDaoImpl bean to merge the attribute source(s). The documentation in Confluence [2] about this should be migrated into this (or the appropriate) repo.

[1] https://github.com/uPortal-Project/uPortal/blob/HEAD/docs/implement/user-attributes/README.md
[2] https://apereo.atlassian.net/wiki/spaces/UPM43/pages/103948916/JDBC+User+Attribute+Sources

Additional context
This came to light via https://groups.google.com/a/apereo.org/g/uportal-user/c/boXVnfKEZAw/m/tQiqv19nAQAJ