mandoc: Vendor import of upstream at 2025-04-08

Interesting changes:
+ correct legacy mandoc date typo, reported on our very own bugzilla
+ improve libmandoc manual
+ strengthen recommendations to omit macros from title lines in mdoc(7)
+ improve html5 compliance in html output
+ improve RFC linking in markdown output
+ libmandoc and mdoc manuals have been improved
+ support arithmetic operations in tbl(7) column widths for DocBook
+ define the St -isoC-2023 macro for referencing the C23 spec

Approved by: mhorne (mentor, implicit)
Reviewed by: imp
Discussed with: adrian, bapt, brooks
Closes: https://github.com/freebsd/freebsd-src/pull/1689