docs: Explain phases and plugin execution order

### Current State

Missing or "hard to find" documentation on phases and plugin execution order

**Context:**

I recently opened the issue #13408 about passing information from one plugin to another. I probably would have been able to solve this by myself with the help of a conceptual documentation about:

* The execution phases each request "goes through"
* Overview of the phase a plugin belongs to
* Explanation, how the execution order of plugins in the same phase works (priority)

### Desired State

Documentation on phases and plugin execution order is inclued and easy to find.

**Hint:**

A good starting point for a big part of the required content is this blog post: https://apisix.apache.org/blog/2023/12/14/apisix-plugins-priority-leaky-abstraction/

Start with the linked blog post on APISIX plugin priority and issue #13408 to understand the user-facing gap. Trace the request phases and plugin priority behavior in the APISIX documentation and implementation, then document the phase overview and same-phase execution order in a clearly discoverable location. Done means newcomers can find and use this explanation to understand plugin sequencing.

Written by the indexing model from the issue text.