Comprehensive URI Template syntax guide

Added a new documentation page explaining RFC 6570 URI Template syntax
and its usage throughout Fedify's routing system. The guide covers:

- Different expansion types ({var}, {+var}, {/var}, {?var}, {&var})
- When to use each expansion type
- Common use cases for actors, collections, inbox listeners, and objects
- Common pitfalls like double-encoding with URI identifiers
- Decision guide and troubleshooting tips

Also added cross-references from actor.md, collections.md, inbox.md,
and object.md to help users choose the appropriate expansion syntax
for their identifiers.

Close https://github.com/fedify-dev/fedify/issues/417