[llvm] [Github][CI] Add `doc8` for clang-tidy documentation formatting (PR #168827)

[via llvm-commits]

github-actions[bot] wrote:

<!--LLVM CODE LINT COMMENT: doc8-->


:warning: Documentation linter doc8 found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

```bash
doc8 ./clang-tools-extra/docs/clang-tidy/checks/
```

</details>

<details>
<summary>
View the output from doc8 here.
</summary>

```
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:99: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:100: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:105: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:106: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:107: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:111: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:112: D001 Line too long
./clang-tools-extra/docs/clang-tidy/checks/bugprone/unsafe-functions.rst:126: D001 Line too long
```

</details>


https://github.com/llvm/llvm-project/pull/168827