Add vendor/ to .markdownlintignore

[tomalec]

What?

Add vendor/ to .markdownlintignore
for wp-scripts, to match the documented behavior
https://developer.wordpress.org/block-editor/reference-guides/packages/packages-scripts/#lint-md-docs

Why?

Documentation states:

By default, files located in build, node_modules, and vendor folders are ignored.

How?

Testing Instructions

0. Open a project that has some lint errors in /vendor folder, like https://github.com/woocommerce/google-listings-and-ads/
1. Run wp-scripts lint-md-docs

Before

> wp-scripts lint-md-docs

…
vendor/automattic/jetpack-a8c-mc-stats/README.md:35 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```php"]
vendor/automattic/jetpack-assets/README.md:7 MD001/heading-increment/header-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h3]

Screenshots or screencast

[gziolo]

Code changes look good. Good catch 👍

Any idea why CI checks didn’t trigger?

Would you mind including an entry in the CHANGELOG file similar to:

https://github.com/WordPress/gutenberg/blob/trunk/packages/scripts/CHANGELOG.md#bug-fix

[tomalec]

Any idea why CI checks didn’t trigger?

😕 maybe because I created this PR via GitHub UI?

Would you mind including an entry in the CHANGELOG file similar to:

Done 80ea265

[gziolo]

Thank you for the changelog change.