Publicly document MSBuild security boundaries, implications and best practices

### Background

We need to publicly document the MSBuild security assumptions and recommended best practices to mitigate breaking those assumptions

### Goals

* [ ] Create doc in a private space to create the content before publishing
* [ ] Add the content around current security assumptions and guarantees and recommended best practices
* [ ] After team vetting - add the content to https://learn.microsoft.com/en-us/visualstudio/msbuild/msbuild (vi PR to https://github.com/MicrosoftDocs/visualstudio-docs-pr)