Improve tiered support system and compilation flags introduction. (bytecodealliance#4750)

* docs: update tiered support documentation with detailed tier definitions and feature classifications and improved compilation flags and links for clarity
* update tiered support documentation to clarify feature support levels and enhance readability
  - Revised descriptions for Tier A to specify that features have been used in products.
  - Reformatted notes on testing, maintenance, and support definitions for better clarity.
  - Added a new section for privileged features with detailed explanations.
  - Updated tables for Tier A, Tier B, and Tier C to include additional targets and compilation flags.

Author: lum1n0us