doc: PG 16 relnotes, initial markup
Still need to add links to doc sections.
Backpatch-through: 16 only
Branch
------
REL_16_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/6b86a3e0b421a0e0c9cfa02ddc0525e431379ee4
Modified Files
--------------
doc/src/sgml/release-16.sgml | 547 +++++++++++++++++++++----------------------
1 file changed, 273 insertions(+), 274 deletions(-)