docs: stale links, a wrong reference link, and obsolete files in several guides

[armando-navarro]

Summary

Reviewing the guides under docs/, I found a set of small, user-facing problems: two documentation links point at the retired angular.io site, one guide links to the wrong reCAPTCHA provider page, a logging-level description uses the wrong verb, a few "a/an" grammar slips, and two files are obsolete.

What I found

• Stale angular.io links. docs/compat.md and docs/compat/auth/router-guards.md link to angular.io, the Angular documentation's former domain. It has since moved to angular.dev, and the API-reference link in router-guards.md currently resolves to the archived v17 documentation. Both should point at angular.dev.
• Wrong reCAPTCHA provider link. In docs/app-check.md, the "Getting Started" link points at the reCAPTCHA Enterprise page, but the code sample uses ReCaptchaV3Provider. The link should point at the reCAPTCHA v3 page.
• Incorrect wording. In docs/zones.md, the "Warn" logging-level bullet says high-risk APIs "are called" where it means they "are logged".
• Grammar. docs/auth.md, docs/analytics.md, and docs/app-check.md read "Provide a Auth/Analytics/App Check instance". It should read "an".
• Obsolete files. docs/firebase.json is a dead Firebase Hosting config for a documentation site that no longer exists, and docs/install-angular-cli-windows10.md is a Windows-10-era CLI install guide that recommends the npm-deprecated windows-build-tools and links the old angularfire2 repository. Both are orphaned (nothing in the docs links to either).

Scope

Documentation only. A pull request follows.