Onboarding docs with contribution guidelines

[sabinevidal]

Adding a contributions section to the docs website to outline how and where people can contribute.

Still to do:

• [x] copy for the sections
• [ ] Onboarding workflow with Swagger?
• [ ] Guidelines for API integration
• [ ] Links to issue template and contribution guidelines/policy
• [ ] Link to docs on kindly-website

[sabinevidal]

@nathanfletcher @jwflory: I have added more info into the overview. I want to flesh out the docs overview a little bit more as this would likely be a first stopping point for new contributors. I thought we could include the contribution guidelines (mentioned in #125) on the api maintenance page. Would appreciate your thoughts when you have a chance, on where I can add more or less detail. Thanks!

[nathanfletcher]

@sabinevidal it seems the docs check failed because of a url https://github.com/unicef/kindly/runs/5067906279?check_suite_focus=true#step:4:36

I think it looks okay but I'd like to hear what Justin also thinks of the changes

[sabinevidal]

@nathanfletcher: Here's the PR for the contribution docs updates!

[sabinevidal]

@nathanfletcher @jwflory Is there anything else I need to add to the docs from updates the last month? Also need to add the link + info for the SwaggerAPI as well as the guidelines/code of conduct for contributions too.

[jwflory]

@nathanfletcher Is this PR ready for final review?