doc: add missing docs & fix some errors in doc testing 📝

[kareemmahlees]

closes #122

Changes

• Clarify how to register custom types.
• Clarify how to register constants
• Trying to pass doc tests
• Add examples for Builder methods.

[kareemmahlees]

This should wrap up the difficulties I encountered while migrating.
But, literally all doc tests fail with errors similar to the following:

---- src\builder.rs - builder::Builder<R>::events (line 151) stdout ----
Test executable failed (exit code: 0xc0000139).

---- src\builder.rs - builder::Builder<R>::commands (line 126) stdout ----
Test executable failed (exit code: 0xc0000139).

---- src\builder.rs - builder::Builder<R>::constant (line 212) stdout ----
Test executable failed (exit code: 0xc0000139).

---- src\builder.rs - builder::Builder<R>::export_str (line 279) stdout ----
Test executable failed (exit code: 0xc0000139).

---- src\builder.rs - builder::Builder<R>::ty (line 188) stdout ----
Test executable failed (exit code: 0xc0000139).

I cannot detect a reason, can you confirm that this is also the case on your end and it's not my crazy Rust setup doing its thing again? @oscartbeaumont

[oscartbeaumont]

Really love these changes!

I have made some changes and left some comments around them (nothing you need to fix). I would love some feedback on my changes before merging!

I also did some messing around and managed to get the doc test to build with tauri::generate_context by copying exactly what Tauri do in their examples.

and turns out generic event's was never implemented but shown in the docs so also fixed that so the doc compiled.