[Doc][FAQ] Add a doc introducing how to use DevLake in different Git flows

## Documentation Scope
There're different branching flows such as
- [Git flow](https://nvie.com/posts/a-successful-git-branching-model/)
- [trunk-based flow](https://www.toptal.com/software/trunk-based-development-git-flow)
- [GitHub flow](https://www.geeksforgeeks.org/git-flow-vs-github-flow/)
- GitLab flow
- etc.

We should tell users
1. How to use DevLake according to their dev practices.
2. How to use DevLake with different 'Merge modes', e.g., merge, merge and rebase, merge and squash, etc.
3. The timing of `deployments` will affect the way to measure DORA with DevLake

## Describe the Change
We should start from
- [ ] guide for Git Flow
- [ ] guide for the trunk-based flow