[EPIC] Help users create and use plugins

[joelanman]

What

Plugins let users share common code such as components, templates and filters. It's a powerful feature but we need to document it and make sure users can easily create, share and use them.

Why

We need to make sure people can easily create plugins, if not the feature won't get used or used as much as it could be.

Done when

Better interface

• [ ] https://github.com/alphagov/govuk-prototype-kit/issues/740
• [ ] #2051
• [ ] #1985
• [x] #2125
• [x] #2242
• [ ] #1705
• [ ] https://github.com/alphagov/govuk-prototype-kit/issues/1866

Non GOV.UK prototypes

• [ ] #2166
• [ ] #2167

User research

• [x] #2156
• [ ] https://github.com/alphagov/govuk-prototype-kit/issues/751
• [ ] #2176

Assurance

• [ ] #2155
• [ ] #2154

Plugin authors

• [x] https://github.com/alphagov/govuk-prototype-kit-docs/issues/188
• [x] https://github.com/alphagov/govuk-prototype-kit/issues/1635
• [ ] #2148 (low priority, will include if there's time)

[BenSurgisonGDS]

I think it would be easy to create a skeleton plugin from within the manage-prototype pages that could then be treated as a separate project. We could even initialize git for it.