doc: distinguish fixed and parametric protocol constants

Context

The current documentation of protocol constants suffers from several issues:

• it is placed in a weird place: in the proof-of-stake page
• it refers to an incomplete list of constants (module Alpha_context.Constants), e.g.
  • missing fixed constants max_micheline_bytes_limit, max_micheline_node_count, max_allowed_global_constant_depth, which are however listed by RPC /context/constants
  • (doesn't seem to miss parametric constants)
• it does not distinguish fixed and parametric constants.

This MR is a followup of a previous discussion and implements a suggestion by @eugenz: !4901 (comment 902474846).

Manually testing the MR

Build the doc or check the CI doc artifact, and verify that it solves the above issues, i.e.:

• it refers to the complete list of protocol constants
• it correctly defines fixed and parametric constants
• it is clear and informative.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR