Document OVERRIDES_CMD_VERBOSE env var for API Fuzzing and DAST API

What does this MR do?

Documents new support to the new variable _OVERRIDES_CMD_VERBOSE. This variable is available in API Fuzzing as FUZZAPI_OVERRIDES_CMD_VERBOSE and DAST API as DAST_API_OVERRIDES_CMD_VERBOSE.

• Document new variable _OVERRIDES_CMD_VERBOSE in API Fuzz and DAST API. The new variable _OVERRIDES_CMD_VERBOSE allows the users to show or hide the output of external commands. The external command is provided by _OVERRIDES_CMD (already documented).
• Document variable _PRE_SCRIPT. The variable was already available in the application, but the documentation was not present. The documentation has been included in this MR because the example provided for _OVERRIDES_CMD_VERBOSE is complemented with the use of the variable _PRE_SCRIPT.
• Document _POST_SCRIPT. The variable is not documented, a simple reference to it has been added in listed variables.

In the related MR by default output of the command is hidden, unless an error is detected. However, if DAST_API_OVERRIDES_CMD_VERBOSE is set to any value (but empty), then the output of the comment is shown as part of the job output.

Additionally, this MR adds an example of how to produce a log file related to the override command.

Related issues

#334578 (closed)

Depends on MR

https://gitlab.com/gitlab-org/security-products/analyzers/api-fuzzing-src/-/merge_requests/537

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