move modules from kebab-case to snake_case (#14439)

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### What does this PR try to resolve?

It is currently unclear reading the docs how files should be named. [RFC
430](https://rust-lang.github.io/rfcs/0430-finalizing-naming-conventions.html)
is clear on crates and module names, `kebab-case` is never mentioned and
while there are historical considerations, we should guide new
development towards `snake_case`. We should align the documented
defaults to that RFC.

### How should we test and review this PR?

Review the docs and determine if we view value in the change.

Author: epage

src/doc/src/guide/project-layout.md

@@ -48,6 +48,8 @@ files, place a `main.rs` file along with the extra [*modules*][def-module]
 within a subdirectory of the `src/bin`, `examples`, `benches`, or `tests`
 directory. The name of the executable will be the directory name.
 
+> **Note:** By convention, binaries, examples, benches and integration tests follow `kebab-case` naming style, unless there are compatibility reasons to do otherwise (e.g. compatibility with a pre-existing binary name). Modules within those targets are `snake_case` following the [Rust standard](https://rust-lang.github.io/rfcs/0430-finalizing-naming-conventions.html).
+
 You can learn more about Rust's module system in [the book][book-modules].
 
 See [Configuring a target] for more details on manually configuring targets.