pgsql: doc PG 17 relnotes: add links to documentation sections

doc PG 17 relnotes: add links to documentation sections

Also slightly improve markup instructions. Indentation is still needed.

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5ade0b8f80cdbb715bc3904ac7d4281a75817911

Modified Files
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doc/src/sgml/release-17.sgml | 523 +++++++++++++++++++++----------------------
doc/src/sgml/release.sgml | 8 +-
2 files changed, 257 insertions(+), 274 deletions(-)