Docs: mention `--locked` source install fallback for Dioxus CLI

I hit this while following the Getting Started CLI install docs on Ubuntu 22.04.

First, the prebuilt install path completed, but `dx doctor` could not start:

```text
dx: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.39' not found (required by dx)
```

Found related issues in the main repo:

- https://github.com/DioxusLabs/dioxus/issues/5081
- https://github.com/DioxusLabs/dioxus/issues/3661

Then I tried the documented source install fallback, using `--force` to replace the broken binary:

```shell
cargo install dioxus-cli --force
```

That also failed:

```text
no associated function or constant named `credential_helper` found for struct `Cred`
```

Found related issues in the main repo:

- https://github.com/DioxusLabs/dioxus/issues/5572
- https://github.com/DioxusLabs/dioxus/issues/5577

Based on the maintainers' comments in these issues, the command that finally worked for me was:

```shell
cargo install dioxus-cli --force --locked
```

While solving this, I noticed the main DioxusLabs/dioxus README already uses `--locked` for source installation from git:

```shell
cargo install --git https://github.com/DioxusLabs/dioxus dioxus-cli --locked
```

Could the Getting Started docs either use `--locked` for the source install fallback, or add a short Linux troubleshooting note for this prebuilt glibc failure case?

That would make the working path discoverable from the docs, so users in this situation do not have to piece it together from multiple issues.

Start from the Getting Started CLI installation documentation and compare its source-install fallback with the working `cargo install dioxus-cli --force --locked` command and the existing Dioxus README example. Update the fallback or add the Linux glibc troubleshooting note so users can find the working path; verify the rendered documentation shows the command clearly.

Written by the indexing model from the issue text.