[clang-tools-extra] [clang-tidy][docs] Rename modernize check docs to Markdown [1/4] (PR #232206)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

<!-- acp-stack:xqa435 -->
**Stack** (1 PR, base `main`), merge from the bottom up:

- `main`
  - **#232206 [clang-tidy][docs] Rename modernize check docs to Markdown [1/4]** (draft) ← this PR

_This stack is still in progress, more PRs may be added._

<sub>Managed by [acp](https://github.com/vbvictor/acp). This comment is updated automatically.</sub>

https://github.com/llvm/llvm-project/pull/232206