Rewrite the consumer docs in the plain-docs voice #164

Rewrite every consumer page in the plain-docs voice (#163), in one change: plain, friendly, example-first and exact.

The pages:

• the root README.md
• src/EFCore.ComplexIndexes/README.md, src/EFCore.ComplexIndexes.PostgreSQL/README.md, src/EFCore.ComplexIndexes.SqlServer/README.md (the NuGet landing pages)
• docs/postgresql-indexes.md, docs/postgresql-constraints.md, docs/sqlserver.md, docs/upgrading.md

A rewrite changes the voice, not the promises. Each page gets a claim-by-claim review against the code before it lands: every condition, exception, SQLSTATE, provider limit and version note survives, and every new sentence matches the source. A statement found wrong on the way is corrected and listed in the PR.

• Every code block stays byte for byte the same.
• Every heading something links to keeps its anchor: the changelogs and published release notes link blob/main/…#anchor, and DocumentationLinkTests checks them. Other headings may become tasks, with every link updated.
• Packed READMEs keep absolute links only.
• The Documentation table in the root README matches the pages' new openings.

Lands after the open 5.5.0 doc changes. #149, #159 and #162 edit docs/upgrading.md, and #159 edits docs/postgresql-constraints.md. Whichever merges second rebases; the rewrite carries their sentences over in the new voice.

Done when the eight pages are rewritten, the unit suite passes, and a script shows every fenced block unchanged against main.