Additional GitHub best practice guidance

As a member of the data insights and statistics division, I am looking for a clear page of best-practice guidance of how we use Github in the division and the department.

Suggestion is to add clear guidance on best-practice for using GitHub in DISD/DfE, that would be a good introduction to a new-starter and tell them everything they need to know, including how we suggest setting up an account, what type of thing can and cannot be stored on there etc.

It should include:

- How to set up accounts
- What type of content should be stored on Github
- How we use GitHub with Git (best practice on committing, pushing and **pull requests**)
- Clear explanations that everything on GitHub is public & the secure alternative for sensitive info (DevOps)

Start by locating the existing guidance pages in analysts-guide and confirm the DISD/DfE policies for account setup, acceptable GitHub content, Git and pull-request practices, public visibility, and the secure DevOps alternative. Done means a clear new-starter page covers each requested topic and distinguishes public information from sensitive material.

Written by the indexing model from the issue text.