Corrects code sample to be able to compile

[glombek]

📋 Description

Corrects code sample to be able to compile. Updated flag provider implementation to include additional using directives and modified the ShouldAddFlag method to be generic.

📎 Related Issues (if applicable)

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

v17.0.0 and some versions of 16 (I think 16.4)

Deadline (if relevant)

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[AndyButland]

Thanks @glombek - this corrects the code sample.

But it's revealed something very annoying - you may have seen but at some point in development we migrated from the name Sign to Flag, and seems we missed the sign providers collection. So maybe we'll look to correct that again, hopefully backward compatibly.

Meantime this can be merged in and I'll provide another update if we do fix this example where the docs were correct and the code was wrong!

[AndyButland]

PR to amend this in code is here: umbraco/Umbraco-CMS#20752, so likely from 17.0-rc2 the correctly named extension method FlagProviders will work again.

[AndyButland]

And PR to correct the docs moving forward is here: #7594