build: add prettier to update-rules-docs script

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Hi,

In this PR, I added Prettier to the update-rules-docs script.

This change was motivated by the implementations in eslint/json and eslint/css:

• https://github.com/eslint/json/blob/main/tools/update-rules-docs.js#L103-L113
• https://github.com/eslint/css/blob/main/tools/update-rules-docs.js#L103-L113

Other plugins like JSON and CSS use Prettier when running the update-rules-docs script, but the Markdown plugin didn't.

It felt like a minor inconsistency across the plugin ecosystem, and since prettifying README.md made it clearer to me, I decided to make this change.

I also removed README.md from .prettierignore so its formatting will be checked in CI.

The changes in README.md were auto-generated by Prettier, and I reviewed them, they only affect formatting.

What changes did you make? (Give an overview)

In this PR, I added Prettier to the update-rules-docs script.

Related Issues

N/A

Is there anything you'd like reviewers to focus on?

N/A

[lumirlumir]

Actually, looking at the JSON and CSS plugins, all of their Markdown files are formatted with Prettier.

• https://github.com/eslint/json/blob/main/.prettierignore
• https://github.com/eslint/css/blob/main/.prettierignore

I'd like to ask how the team would feel about formatting all Markdown files in this repo with Prettier. (If that's out of scope for this PR, I'm happy to open a separate issue.)

The Markdown repository includes code-block linting examples, so if the main concern is those examples, not adding Prettier here seems reasonable to me.

[nzakas]

In theory it shouldn't matter as we should be able to properly lint the Markdown files even after they're formatted. I'd say try formatting them all and then linting. If there aren't any errors (or any errors that aren't easily fixable), then we can move forward with formatting all the .md files.

[lumirlumir]

Thank you for taking a look.

I tried it, but Prettier is more opinionated with Markdown formatting than I expected. (It changes some of the rule examples that we intentionally included.) So, I think it's fine to just format the root-level README.md file as it is now.

[mdjermanovic]

LGTM, thanks!