Docs: Add instructions for installing plugins/themes with wp-env

[AetherUnbound]

Description

This PR adds an example for installing plugins and themes using wp-env. I encountered an issue similar to wp-cli/wp-cli#3255, docker-library/wordpress#417 where I was unable to install plugins or themes. I've provided instructions for how to install plugins using the conventional wp-env method, as well as an alternate in case that does not work for some folks.

How has this been tested?

The additional command I provided was the only way I was able to install a plugin locally; I've tested that both themes and plugins can be installed with this method.

Screenshots

Types of changes

Small docs addition!

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

(I've left these unchecked since they don't appear to apply to documentation, but please let me know if there are any I should check/run)

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @AetherUnbound! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[Mamaduka]

Thanks for working on this, @AetherUnbound.

Let's merge this PR once all checks are green ✅

[AetherUnbound]

Fantastic, thanks for the quick turnaround! 🙌🏼

[oxyc]

Trying to run wp-env on Github Actions and running into this but the workaround doesn't fix it either.