sql-statements: use EBNF to render syntax diagrams for ADD, ALTER and ANALYZE statements (#4722)

[ti-srebot]

cherry-pick #4722 to release-2.1
You can switch your code base to this Pull Request by using git-extras:

# In docs repo:
git pr https://github.com/pingcap/docs/pull/4764

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs.git pr/4764:release-2.1-4c603f53ce62

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What is changed, added or deleted? (Required)

This PR changes the synopsis sections of ADD, ALTER and ANALYZE statements to use the ```ebnf+diagrm block, replacing the original image-based sqlgrams.

To avoid making one single PR too large, I will open 4-5 follow-up PRs to process sql-statements files in batches. And I will delete all image files in the last PR in case of any display issues.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: sql-statements: use EBNF to render syntax diagrams for ADD, ALTER and ANALYZE statements docs-cn#5324
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@CharLotteiu you're already a collaborator in bot's repo.

[TomShawn]

LGTM