Move Advanced NGINX Extensions proposal to Implementable

Problem:
- Advanced NGINX Extensions proposal nginx#2035
  is in Provisional state - only problem statement is defined.

Solution:
- Add the implementation details to the proposal.
- Move the enhancement proposal to Implementable

Notes:
- The goal of Allowing users to customize supported NGINX configuration
  was moved to a Non-Goal to keep the size of the proposal manageable.
- This enhancement proposal can be further split into two - one for
  SnippetsPolicy and one for SnippetsTemplates

CLOSES - nginx#815

Author: pleshakov