_content/doc: change relnote definition lists to headings

Since 1.7, the "Minor changes to the library" second has used
a definition list to describe the changes to each package.

Use level 4 headings instead. This removes one feature
that is not CommonMark-compliant. It paves the way for
using a CommonMark-compliant Markdown parser and renderer
for both display and generation of release notes.

For golang/go#64169.

Change-Id: I2327499971a2db4eee06ef35e3e41cd0d2f953b3
Reviewed-on: https://go-review.googlesource.com/c/website/+/541976
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: jba