Add developer guide

[edwardchalstrey1]

Adding an initial development guide to the project documentation.

This PR:

• Closes Seshat project GitHub workflow proposal & timeline edwardchalstrey1/seshat#176
• Covers how to use issues, project board and the GitHub workflow for Seshat
• Does not include how to update the live website - I have added a placeholder link TODO for another page that can explain that once we have determined exactly how it will work in the long run

Reviewers should:

• Check the workflow makes sense and comment if anything is unclear or should change
• @MajidBenam note: you can build the docs locally or just look at the Files changed tab here in the PR
• Question for @kallewesterling - are the docs live on GitHub pages already? If not, would be good to enable that and also have a docs preview link to generate on a PR

[kallewesterling]

They are live already! See deployments here: https://github.com/Seshat-Global-History-Databank/seshat/deployments/github-pages

And here's the GitHub rendering: https://seshat-global-history-databank.github.io/seshat/

[kallewesterling]

Perhaps we should keep TODOs in comments?

The way to do that in RST is:

..
    The comment goes here.

[kallewesterling]

Otherwise all good, I think!

[edwardchalstrey1]

@kallewesterling I also removed info from the README that is now duplicated in the docs