Add page for contributing to the guide

[lb-]

Flagging that the link to help improve this guide does not go anywhere yet.

We should add this page and also call out this Guide contribution on the main Wagtail docs above 'Other contributions'.

https://docs.wagtail.org/en/stable/contributing/index.html#other-contributions

Additional to providing details on how to translate, we should really provide details on how to create an account and start contributing #67

[thibaudcolas]

@lb- thanks LB. The site is still under development with partly implemented front-end, content, back-end – so it’s expected for now that there will be gaps in implementation. And right now we’re not at a place where we can solicit contributions yet.

[lb-]

Makes sense. Thanks Thibaud

[thibaudcolas]

I’ve made a very simple page for the time being: https://guide.wagtail.org/en-latest/contributing/

Will ad this to the footer once #78 is done.