doc: introduce validation modes

Context

Closes #2536 (closed).

This MR aims implementing the consensual decision in a thread of the above MR: #2536 (comment 871450714). It should introduce the concept of validation modes both the Tezos users and to core developers.

Manually testing the MR

Check that the provided explanations introduce the concept of validation modes and are useful and clear.

Checklist

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