Auto merge of #54733 - GuillaumeGomez:stabilize-rustdoc-theme, r=ollie27,Dylan-DPC

Stabilize rustdoc theme options

Closes #54730

This PR stabilizes the `--themes` (now `--theme`) and `--theme-checker` (now `--check-theme`) options, for allowing users to add custom themes to their documentation.

Rustdoc includes two themes by default: `light` and `dark`. Using the `--theme` option, you can give rustdoc a CSS file to include as an extra theme for that render. Themes are named after the CSS file used, so using `--theme /path/to/your/custom-theme.css` will add a theme called `custom-theme` to the documentation.

Even though the CLI flag to add a theme is getting stabilized, there's no guarantee that a theme file will always have the same effect on documentation generated with future versions of rustdoc. To aid in ensuring that a theme will work, the flag `--check-theme` is also available, which compares the CSS rules defined by a custom theme against the ones used in the `light` theme. If the `light` theme defines a CSS rule that the custom theme does not, rustdoc will report an error. (Rustdoc also performs this check for themes given to `--theme`, but only reports a warning when a difference is found.)

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