add the rsa generating command in check-before-deployment.md (#5200)

[ti-srebot]

cherry-pick #5200 to release-5.0
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/5216

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

git push git@github.com:ti-srebot/docs.git pr/5216:release-5.0-267c628ce09c

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• 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: Update check-before-deployment.md docs-cn#5823
• 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]

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

[TomShawn]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: e78f1d8