Add best practices for project/repo setttings

[inkydragon]

Whenever a new project/package is created. We need to do a lot of configuration of infrastructure settings.
A recent example is: JuliaLang/LinearAlgebra.jl#1115
In the above pr we want to set up DependaBot, but the defaults may not be suitable, so some changes need to be made.

There are many more similar best practices when configuring the project CI:

• Specify the commit id of the action instead of the version number for better security
  CI: Use Dependabot to update GitHub Actions in this repo, and pin all external GitHub Actions to full-length commit hashes julia#55283
• macOS -x64 with aarch64
  How to fix GitHub Actions CI failures with Julia 1.6 (or 1.7) on macos-latest and macos-14 - Tooling / Package Management - Julia Programming Language
• ...

I think oh devguide is a good place to write them down.