Plugins: Add a resource that teaches people what plugins are and how to use them

This issue was ported over from WordPress Slack.

"how to use plugins" tutorial: i don't know if it should be on the top or the bottom (although my preference is for it to be on the top), but there needs to be a beginner-friendly tutorial on "what are plugins and how to install, use and update them". i vividly remember the first time i downloaded a plugin, i saw the zip file and unzipped it, and couldn't figure out how to install it on my site. i'm sure there are plenty of other users like me. plugins are not at all intuitive, which is fine, but that's why documentation is important.

• it's surprising that the only tutorials available on this page is about adding your plugin or creating your plugin.

There is a documentation article all about how to Manage Plugins.

Perhaps we can update the footer, in conjunction with #224 and #257, to include a CTA and link to that resource.

Current Footer

Start by reviewing the current footer and the linked Manage Plugins article, then read issues #224 and #257 for related footer changes. Done means the plugins resource has a beginner-friendly CTA and link in the footer, with the placement decided.