Add documentation for usage of pre/post-build events from the Property Pages

Based on this feedback item: https://developercommunity.visualstudio.com/t/Almost-impossible-to-use-post-build-even/10211397

It seems there is general confusion around the usage of pre-build and post-build events. In that feedback, the user calls the `powershell.exe` and issues individual PowerShell statements per call. It isn't a very efficient way to use this feature. We've also made it a bit harder to understand from a project-system side as now we collapse all lines into a single line in the `.csproj` file. This change was done [here](https://github.com/dotnet/project-system/pull/7119) to resolve issues accessing common MSBuild properties. For example, previously you'd have:
```xml

echo Line 1
echo Line 2
echo Line 3

```

Is now:
```xml

```

This becomes especially difficult to read for long scripts. We should clarify:
- How the script is executed
- What working directory it is executed in
- What is available (MSBuild properties, environment variables, etc.)
- Any other relevant information

I left a somewhat detailed explanation in the feedback item on the specific scenario the user was experiencing related to double-quotes and the PowerShell CLI. [Here](https://github.com/PowerShell/PowerShell/issues/11747) is a close-*ish* issue to what was experienced in the feedback from the PowerShell side of things.

Overall, determine if we can provide clarity on how to pre/post-build. I'm not sure if this is our team's responsibility or MSBuild or SDK or whomever, but it should be addressed somewhere.