Update source code create backend handbook page
### Why
The [handbook](https://handbook.gitlab.com/handbook/engineering/development/dev/create/source-code-be/) should be the SSOT for any relatively stable information about the team: processes, channels, practices etc. Currently it is out of date, contains broken or outdated links, and is missing information.
It could be so much more. This team page is really a chance for us to showcase our ability to organize our information, share our team culture, and share how we incorporate our CREDIT values as a team. It could drive a better Result especially by leveraging Efficiency and Transparency. We should be able to find everything we need on this page that's related to any team concepts.
Having our handbook page be up to date and relevant is _key_ to driving a handbook first, transparent, async culture where everyone can contribute.
### What
This handbook page won't ever be perfect, and we should avoid one huge effort to "fix everything."
Instead, we should foster a culture of "Everyone Can Contribute" and make keeping our handbook up to date a part of our practice.
- We should be merging at least 1 handbook MR a week until it becomes difficult to answer "what else needs to change" on the page.
- Individual issues are broken out to encourage small MRs with high participation from the team.
- The whole team should be pinged for review on each MR. Participation of course will vary with interest but everyone should have the opportunity to contribute.
epic