fmt: split package documentation into more sections

The package-level documentation on fmt previously had only two formal
sections, for printing and scanning. Because of this, the section on
printing was very long, including some pseudo-sections describing
particular features. This feature makes those pseudo-sections into
proper sections, both to improve readability and so that those sections
have hyperlinks on documentation sites.

Fixes #46522

Change-Id: I38b7bc3447610faca446051da235edcbbd063f61
Reviewed-on: https://go-review.googlesource.com/c/go/+/324349


Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>