Add Axum activity feed example

[VimCommando]

Expanding on the "Hello World" example, this activity feed includes additional implementations.

From the axum-activity-feed.html:

1. Nine data-signals
2. Two data-bind for inputs
3. Four data-on-click buttons
4. A conditional data-class for disabled button styling
5. Seven dynamic data-text elements

From the axum-activity-feed.rs:

1. A multi-value Signals struct
2. A Status enum that is also used as an extract::Path route property
3. A long-running generate function that emits a user-defined number of events to the SSE stream
4. An event function that emits a single signal/element pair