doc: update EXPLAIN PLAN docs

[aalexandrov]

Update the docs to add the newly supported syntax introduced in some recently merged PRs (#21708, #21973, and #22021).

Motivation

• This PR adds a known-desirable feature.

We want to document the EXPLAIN syntax extensions that landed in the past weeks (they are all live in prod as of today).

Tips for reviewer

The changes are exclusively in the EXPLAIN PLAN docs page. Here is a direct preview link.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • The EXPLAIN syntax now supports CREATE MATERIALIZED VIEW and CREATE INDEX statement types as well as a humanized_exprs flag that can be used to produce more readable EXPLAIN AS TEXT output.

[morsapaes]

Thank you, @aalexandrov!