doc: Add Features documentation

Add Features.md with complete overview of OTP capabilities organized into
logical categories. Integrate the features page into documentation navigation and add cross-references from key documentation pages.

Changes:
- Add doc/user/Features.md with comprehensive feature list
- Update mkdocs.yml to include Features in About section
- Add links to Features.md from index, Product-Overview, Configuration,
  RoutingModes, and Basic-Tutorial pages
- Update Transit Group Priority description

Author: t2gran

doc/user/Basic-Tutorial.md

@@ -205,3 +205,10 @@ You can run the OTP .jar file with the `--help` option for a full list of comman
 ## Exploring the API
 
 If you want to learn how to use OTP's API's, check out the [GraphQL tutorial](apis/GraphQL-Tutorial.md).
+
+## Next Steps
+
+Now that you have OTP running, you may want to explore:
+- The complete [list of OTP features](Features.md) to understand all capabilities
+- [Configuration options](Configuration.md) to customize your deployment
+- [APIs documentation](apis/Apis.md) for integrating OTP with your applications

doc/user/Configuration.md

@@ -215,7 +215,7 @@ Features that can be toggled in this file are generally only affect the routing
 but for consistency all such "feature flags", even those that would affect graph building, are
 managed in this one file. 
 
-### OTP Features
+### OTP Feature toggles
 
 Here is a list of all features which can be toggled on/off and their default values.