Clarify some manifest values and add examples

[sredna]

📖 Description

• Tries to clarify the description of some of the manifest values.
• Adds value examples.
• Adds the UpgradeCode definition.

Notes:

• UpgradeCode is restricted only to GUIDs because this is a MSI feature. I have verified that all existing manifests in the community repository only uses GUIDs here.
• DefaultInstallLocation is restricted to strings starting with % or \. The thinking is that the only sane values are %ProgramFiles%\MyApp, %LOCALAPPDATA%\Programs\MyApp and \AppInRootOfDrive.

🔗 References

Fixes some of the issues reported in #6221

🔍 Validation

None/YOLO

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

[JohnMcPMS]

The changes seem reasonable to me, but I don' actually touch the schema much so I would like someone else to take a look too.

The flow for semantic changes to the schema as part of a new minor version also requires quite a bit more setup. See #5997 and/or #5011.

I think I will also create some additional documentation to prevent the need for you to parse through those if you want to wait for that.

[JohnMcPMS]

#6280 ; you could either grab the script from that PR or wait for it to be merged. But #6279 is updating the minor version that the script picks up so probably want that too.