[DOCS] Improve documentation for Cargo audit configuration file

[p21nc3]

Fixes #818

Description

Cargo audit provides flexible controls for which kinds of advisories are reported (e.g. show/hide unmaintained or unsound) and which are considered fatal (i.e. cause non-zero exit code, cause CI to fail) via a config file is to be located in .cargo/audit.toml. However, this file is not discoverable, leading to people complaining that the default doesn't work well for them and asking to change the defaults.

This pull request proposes to add the following to the Cargo audit documentation:

• A section on the configuration file, including its location and a brief description of the settings that can be configured.
• A link to the TOML documentation, which provides more detailed information about the TOML format.

This will help users find and understand the configuration file, and make it easier for them to customize the behavior of Cargo audit.

Changes

• Added advisories (unmaintained, unsound and critical) in .cargo/audit.toml file.
• Added a section on the configuration file to the Cargo audit documentation.
• Added a link to the TOML documentation to the Cargo audit documentation.

[amousset]

The .cargo/audit.toml file in the repository is the file used for the project itself, and is not a documentation file. Documentation (like the line above does) should point to the example file audit.toml.example.

[amousset]

I don't see where these option names come from, the advisories config section looks like it does not include them.