Fix documentations inaccuracies and missing features

[radcortez]

Fixes:

• smallrye.config.mapping.validate-unknown default is true, not false
• converters/custom.md: service file listed CustomValue instead of CustomValueConverter
• mappings.md: duplicate variable any and missing method-call syntax in @WithDefaults example

New content:

• mappings.md: document @ConfigMapping beanStyleGetters attribute
• mappings.md: document @WithUnnamedKey eager attribute
• mappings.md: document withValidateUnknown() builder API
• mappings.md: document withMappingIgnore() path patterns (exact, *, **)
• config-sources/factories.md: document ConfigurableConfigSourceFactory with full example
• secret-keys.md: document custom SecretKeysHandler, SecretKeysHandlerFactory, and LazySecretKeysHandler
• extensions/interceptors.md: document iterateNames() interception point
• extensions/interceptors.md: document ConfigSourceInterceptorContext.proceed() vs restart()
• extensions/interceptors.md: align example priority with Priorities.APPLICATION recommendation