github(both): improve README.md and simplify PR templates

[Rustin170506]

What problem does this PR solve?

Issue Number: ref #3194

What is changed and how it works?

• improve README.md, TiCDC currently does not support cdc-log
• The PR template is too complex and difficult for people to fill in, so simplify it.

Check List

Tests

• No code

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• buchuitoudegou
• overvenus

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[Rustin170506]

/merge

[ti-chi-bot]

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

Commit hash: 4b93324

[ti-chi-bot]

@hi-rustin: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.