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/templates/RoutingModes.md

@@ -1,6 +1,6 @@
-This page is intended as an exhaustive listing of what OTP's routing engine is capable of and 
-therefore documents internal names. Since OTP has multiple APIs where each works slightly differently,
-please consult your API documentation on how to select the appropriate mode.
+This page is intended as an exhaustive listing of what OTP's routing engine is capable of and
+therefore documents internal names. Since OTP has multiple APIs where each works slightly 
+differently, please consult your API documentation on how to select the appropriate mode.
 
 <!-- INSERT: street-modes -->
 

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