Show compatible targets within docs and gallery

[Arlodotexe]

          > One thing we'll have to think about is how we mark which platforms are supported within the docs and that script. Like for now for WASM it's easiest to remove the markdown files, but we may want to modify this approach to inject the supported platforms into the doc and possible just turn-off sample compilation for unsupported platforms or something.

The script linked is for modifying the documents before exporting for publishing on learn.microsoft.com. It replaces our custom ![SAMPLE_NAME] syntax with the content of the sample and updates all learn.microsoft.com links to be relative.

We want to display which MultiTarget or TargetFramework a component is supported on from the learn.microsoft.com side of things for our users. Would this be as inline text, as metadata, or something else? This would be in addition to the information available on the "Frameworks" tab shown on nuget.org.

We'll need to also display this information in the header for the sample gallery. For display purposes, we should avoid doing this inline. That means we'll need the following:

1. The DocumentationRenderer needs to identify what MultiTargets are supported for the component a given document is contained in, and display it in the header.
2. The doc export script (the one linked, not yet in the repo) provided will need to pull from MultiTarget.props and inject it as front-matter, which can be consumed and rendered by docfx. We can extract/reuse existing code in GenerateMultiTargetAwareProjectReferenceProps.ps1 for this, but it's not yet clear how we'd display this on the learn.microsoft.com side of things.

Once we have these, we can display the supported MultiTarget or TargetFrameworks for a component within both the sample gallery and the published documentation.

Originally posted by @Arlodotexe in #141 (comment)

[agneszitte]

Leaving a little comment here for better context regarding the non-Windows platform compatibility for the documentation for WCT packages:

@Arlodotexe regarding Uno Platform, as we discussed previously here on Discord, displaying MultiTargets or TFMs alongside package names in the ⁠community-toolkit gallery app would be a very good idea. Additionally, having this information in the WCT documentation and/or README of the CommunityToolkit packages would be helpful when browsing nuget.org info.

Once this information is added to the WCT gallery app and/or documentation and/or README for the packages, please let us know. We can then include this info in our documentation, making it easier for users to quickly find the information based on the components they want to use.