[3.12] Docs: format sys.float_info properly (GH-108107) (#108130)

Docs: format sys.float_info properly (GH-108107)

- Normalise capitalisation and punctuation
- Use attribute markup for named tuple attributes
- Use :c:macro: markup for C macros
- Use a list for the 'rounds' attribute values
- Use list-table, for better .rst readability
- Remove one unneeded sys.float_info.dig link

(cherry picked from commit ca0c6c1f1ef79d10bc49b61d638d87cde265aa94)

Co-authored-by: Erlend E. Aasland <erlend@python.org>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>