style-guide: Add try blocks

[ia0]

The tracking issue for try_blocks is #31436.
The tracking issue for try_blocks_heterogeneous is #149488.

I'm not sure if this should be in nightly.md instead.

See #t-lang > try blocks @ 💬 for context.

[rustbot]

Some changes occurred in src/doc/style-guide

cc @rust-lang/style

[rustbot]

r? @calebcartwright

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