Fix documentation issues for 4.x

[dereuromark]

Summary

First Pass

• Update AbstractIdentifier::CREDENTIAL_* to PasswordIdentifier::CREDENTIAL_* (constants moved in 4.x)
• Add missing return types to code examples (initialize(): void, _setPassword(): string)
• Fix broken tutorial link to use absolute CakePHP Book URL
• Replace deprecated strpos() with str_starts_with() (PHP 8+)
• Update API link from CakePHP 4.0 to 5
• Remove misleading identifier config from Session authenticator examples (Session doesn't use identifiers)
• Remove outdated "Added in version 2.10.0" notes that are confusing in 4.x docs
• Add missing use Firebase\JWT\JWT; statement in JwksController example
• Use strict comparison (===) instead of loose (==)
• Add missing MiddlewareQueue type hints in migration guide
• Remove unused ResponseInterface import
• Add missing documentation links in contents.md (redirect-validation, upgrade-3-to-4)

Second Pass

• Remove CONFIG double slash (CONFIG . '/jwt.key' -> CONFIG . 'jwt.key')
• Fix 'explicity' typo -> 'explicitly'
• Remove trailing comma from nonce default value
• Update route syntax to CakePHP 5 ({controller} instead of :controller)
• Remove additional Session authenticator identifier config in middleware.md
• Remove outdated loadIdentifier() references (method removed in 4.x)
• Fix grammar: 'an user' -> 'a user'
• Add return types to IdentityInterface method examples
• Update IdentifierCollection reference to reflect 4.x architecture (now uses single identifier)
• Remove outdated ResponseInterface from docblock parameter
• Use named arguments for finder: find('byToken', token: $token)
• Use string instead of array for request->is('post')

Third Pass

• Add return types to all controller action examples (login(), logout(), beforeFilter(), etc.)
• Fix "Let's" grammar (was "Lets")
• Fix stopImpersonation() typo -> stopImpersonating()
• Replace deprecated TableRegistry::getTableLocator()->get() with fetchTable()
• Fix indentation inconsistency in testing examples
• Add missing redirect return in revertIdentity() example
• Use fetchTable() instead of $this->Users property access
• Use saveOrFail() instead of save() for better error handling
• Use 'plugin' => false instead of 'plugin' => null for consistency
• Add trailing commas in arrays

[josbeir]

maybe wait with this merge after #779 is in because it fixed a bunch of linting errors

[dereuromark]

I would do the other way around. More important to get the wrong Infos fixed i guess