Fix round-tripping of web-snippet and add optional linking to alternate source file

[flcdrg]

• Ensure that content of web-snippet is refreshed when rerun
• Use the URL as the anchor name (like 'snippet' does)
• Add support for optional second URL for web-snippet. If this is provided then it will be used for the source hyperlink URL. So you could use https://raw.githubusercontent.com/flcdrg/aspire-non-dotnet/refs/heads/aspire-complete/AspireAppHost/AppHost.cs as the first URL, but use https://github.com/flcdrg/aspire-non-dotnet/blob/aspire-complete/AspireAppHost/AppHost.cs as the second, which will then get line numbers appended.

[flcdrg]

If you'd prefer, I could pull out the 'second URL' feature into a separate PR