add a circleci integration: check unclosed tags (#3786)

[ti-srebot]

cherry-pick #3786 to release-4.0

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What is changed, added or deleted? (Required)

After we migrate to the new website implementation, unclosed HTML tags will cause a build failure. If we just rely on people to review and find all unclosed tags from possibly large changes, it is not realistic at all.

So I wrote a python script as a new circleci integration, which detects the unclosed tags from all changed files in a PR.

This script can:

1. skip tags in frontmatter because they won't affect build
2. scan all body texts to find all open tags

Note this script cannot yet skip scanning tags from code blocks wrapped by ~~~ ~~~, but our markdown documents use ``` ``` style of code blocks consistently so it should be okay.

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

• master (the latest development version)
• 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:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Have version specific changes
• Might cause conflicts

[ti-srebot]

@yikeke please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs/invitations

[yikeke]

LGTM

[yikeke]

/merge

[ti-srebot]

/run-all-tests