docs: remove duplicated words in reference and guide pages

[eknathdj]

Summary

Remove duplicated words and fix grammar in documentation pages.

File	Fix
reference/extensions_api.html.md	"should must" → "must"; "that that" → "that"
blueprints/staging-prod-isolation.html.md	"a a" → "a"
js/prisma/postgres.html.md	"to to" → "to"
js/prisma/sqlite.html.md	"to to" → "to"

Test plan

• Verified each issue present before editing
• Residual scan clean on changed files