docs: Provide pointers for MSRV

In today's cargo team meeting, we discussed the Pre-RFC for MSRV-aware
resolver.
In that discussion, the question of recommending a policy came up.
While we didn't feel the ecosystem has coalesced enough to set one (and
we hope MSRV-aware resolver will avoid the need),
it became clear that some we can provide some basic help to the user,
including
- Raising awareness of tools to find the actual MSRV
- The policy that they should verify it with examples on how to do so

While this recommends some specific third-party tools,
I'm not aware of other tools within this for us to worry about at this
time for us to create any guidelines on which we should include.

Author: epage

src/doc/src/guide/continuous-integration.md

@@ -156,6 +156,30 @@ jobs:
 For projects with higher risks of per-platform or per-Rust version failures,
 more combinations may want to be tested.
 
+## Verifying `rust-version`
+
+When publishing packages that specify [`rust-version`](../reference/manifest.md#the-rust-version-field),
+it is important to verify the correctness of that field.
+
+Some third-party tools that can help with this include:
+- [`cargo-msrv`](https://crates.io/crates/cargo-msrv)
+- [`cargo-hack`](https://crates.io/crates/cargo-hack)
+
+An example of one way to do this:
+```yaml
+jobs:
+  msrv:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: taiki-e/install-action@cargo-hack
+    - run: cargo hack check --rust-version --workspace --all-targets --ignore-private
+```
+This tries to balance thoroughness with turnaround time:
+- A single platform is used as most projects are platform-agnostic, trusting platform-specific dependencies to verify their behavior.
+- `cargo check` is used as most issues contributors will run into are API availability and not behavior.
+- Unpublished packages are skipped as this assumes only consumers of the verified project, through a registry, will care about `rust-version`.
+
 [`cargo add`]: ../commands/cargo-add.md
 [`cargo install`]: ../commands/cargo-install.md
 [Dependabot]: https://docs.github.com/en/code-security/dependabot/working-with-dependabot

src/doc/src/reference/manifest.md

@@ -185,6 +185,10 @@ The `rust-version` may be ignored using the `--ignore-rust-version` option.
 Setting the `rust-version` key in `[package]` will affect all targets/crates in
 the package, including test suites, benchmarks, binaries, examples, etc.
 
+To find the minimum `rust-version` compatible with your project, you can use third-party tools like [`cargo-msrv`](https://crates.io/crates/cargo-msrv).
+
+When used on packages that get published, we recommend [Verifying `rust-version`](../guide/continuous-integration.md#verifying-rust-version).
+
 ### The `description` field
 
 The description is a short blurb about the package. [crates.io] will display