pd: add pd microservices section

[rleungx]

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[ti-chi-bot[bot]]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot[bot]]

@rleungx: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	5e201243abb93e04de4deb227e91b105839f7f7f	link	true	/test pull-verify

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[qiancai]

Close this PR as its content has been moved to https://github.com/pingcap/docs-cn/pull/15975