chore(clippy): ignore CometBFT as an identifier (#1913)

## Summary
Allows writing CometBFT without backticks in Rust doc comments.

## Background
Clippy's pedantic settings consider CometBFT a Rust identifier because
of its CamelCase. That's annoying, because it's actually the name of the
consensus implementation we are using.

## Changes
- Create a `clippy.toml` in the root of the repository.
- Add `doc-allowed-idents = ["CometBFT"]` to it.

## Testing
Writing `CometBFT` in a Rust doc no longer triggers a clippy warning (no
lines in this PR).

clippy.toml

@@ -0,0 +1 @@
+doc-valid-idents = ["CometBFT", ".."]