Update Blazor Hybrid section

[guardrex]

Fixes #22441

The tracking aspect of #22441 has been superseded by #24956, but we do need this update for the 6.0 release notes on the subject. I'll close out the issue with this update.

Along the way here, let's split out this Blazor Hybrid bit to an H2 section.

WRT the placeholders that I'm 🔪 out here ...

• Blazor WebAssembly packaging: Already covered in the WebAssembly deployment layout section. It was named kind'a funky on the PU roadmap, so it ended up as a dup for coverage here.
• Dynamically added root components: Also from the roadmap. There's a Blazor tracking issue for coverage. I'm dropping the placeholder here. If the tracked issue is addressed by the PU, then I'll circle back here and add something on it.