Обсуждение: pgsql: doc: PG 16 relnotes, initial markup

doc:  PG 16 relnotes, initial markup

Still need to add links to doc sections.

Backpatch-through: 16 only

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6b86a3e0b421a0e0c9cfa02ddc0525e431379ee4

Modified Files
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doc/src/sgml/release-16.sgml | 547 +++++++++++++++++++++----------------------
1 file changed, 273 insertions(+), 274 deletions(-)