Changes

docs: add guide for cherry-picking pull requests to another branch (#3633)

This pull request introduces new documentation for the CI repository,
focusing on guides for CI processes, cherry-picking pull requests, and
improving the overall organization of the documentation. The changes aim
to provide clear instructions and streamline workflows for contributors.

### New Documentation Guides

*
[`docs/guides/CI.md`](diffhunk://#diff-31ffbb8db63aa2ea3d479fc3cf432aa2c40cd6bd4a62eb002f05cb54a84bcab8R1-R64):
Added a comprehensive guide to PingCAP's CI system, including
instructions on locating pipelines, modifying and testing pipelines, and
the workflow for deploying changes from staging to production. Includes
a workflow diagram for clarity.
*
[`docs/guides/cherry-pick-pull-request.md`](diffhunk://#diff-aa61d19c922de9de477cfd5929062498a4e36a42ad2024bce0568b67193b02e0R1-R133):
Introduced a detailed guide on how to cherry-pick pull requests to
another branch, covering prerequisites, usage of the helper script,
handling conflicts, and best practices.

### Improved Documentation Organization

*
[`docs/guides/README.md`](diffhunk://#diff-8d80745b4e642475274e1de3100d73869664e9ed435af8f839eff86b39acbbedR1-R42):
Updated the README to serve as an entry point for the guides section.
Added links to available guides, including CI, Docker build
instructions, cherry-picking pull requests, and FAQs. Provided
recommendations for writing and updating guides.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
 (commit: 9e0f792)