number theory documentation cleanup

[videlec]

The documentation for number theory in the reference manual is a bit confusing. There is a section Number Theory, Algebraic Geometry in the index which lacks many integer related features (which are hidden in the Standard Rings and Fields section).

We also correct some formatting (like #xyz -> :trac:`xyz` or `QQ` -> `\QQ`).

Depends on #18169

CC: @mezzarobba

Component: documentation

Issue created by migration from https://trac.sagemath.org/ticket/18300

[kcrisman]

comment:1

Just be sure not to change links too dramatically - in the index is fine, but we don't want third-party links to our doc to go down without a good reason.