Add link to REML docs to `user-db.html`

[delphidabbler]

While Docs/Design/FileFormats/main-db.html links to REML documentation, user-db.html does link there at all, depsite mentioning REML 28 times!

Rectify this be adding a link to REML docs to user-db.html.

[delphidabbler]

Implemented at commit 5217e27