docs: explain shouldRevalidate and call-site revalidation opt-out

[jasperfurniss]

Summary

The Framework Mode route module page documents shouldRevalidate as return true and links out to the typedoc. That seems like a bit of a step down from the v6 shouldRevalidate page, and it never mentions:

• returning false to skip this route's loader
• that a child cannot skip root / parent loaders (so fetcher.submit() still revalidates the rest of the tree)
• inspecting formMethod / formAction / defaultShouldRevalidate
• call-site defaultShouldRevalidate={false} on Form, Link, useSubmit, fetcher.submit, navigate, and setSearchParams

The call-site API is on the Form and Link reference pages, but it is easy to miss if you start from the route module guide. It was added as unstable_defaultShouldRevalidate in v7.11 and stabilized in v7.15.

Changes

• Fill in docs/how-to/optimize-revalidation.md (it was a hidden stub; #15431 already links to it)
• Expand Framework and Data Mode shouldRevalidate sections with opt-out examples and links to that how-to
• Point Actions, Fetchers, and Resource Routes at the same guidance

Test plan

• Confirm headings and internal links render on a docs preview
• Click through Form #defaultshouldrevalidate and the typedoc ShouldRevalidateFunctionArgs link