[Follow-up] Add first-run onboarding tip for C# Workspace Dev Mode and solution behavior

[TehraniD]

A one-time onboarding tip would help explain:

• What C# Workspace Dev Mode means
• How solution switching affects loaded context
• How to configure dotnet.defaultSolution

Why this helps

• Reduces onboarding friction
• Makes behavior feel intentional and predictable
• Helps users self-serve before filing issues

Acceptance criteria

• One-time onboarding shown on first relevant open
• Includes quick links to setting/docs
• Dismissible and non-intrusive

[dany-on-demand]

The onboarding text claimed you need feedback so here is mine:

Please have a human write the onboarding markdown WBD- hybrid announcement.md. The alarm emoji... the text is utterly embarrassing and makes me lose trust in this institution