[docs] Improve Snyk parser documentation with export instructions and enterprise workflow

[balaakasam]

Description

The existing Snyk parser documentation only contained a one-line description and default deduplication fields. This update expands it to include:

• Overview of supported Snyk report types and which parser to use for each (SCA, SAST, Issue API)
• Step-by-step export instructions from both the Snyk Web UI and Snyk CLI for multiple package managers
• Severity mapping table from Snyk to DefectDojo
• Recommended workflow for enterprise teams running Snyk at scale across multiple applications
• Deduplication guidance for large Snyk imports across multiple projects

Test results

N/A - documentation only change, no code modified.

Documentation

This PR is the documentation update itself, improving the existing Snyk page at ocs/content/supported_tools/parsers/file/snyk.md

Checklist

This checklist is for your information.

• Make sure to rebase your PR against the very latest dev.
• Features/Changes should be submitted against the dev.
• Bugfixes should be submitted against the bugfix branch.
• Give a meaningful name to your PR, as it may end up being used in the release notes.
• Your code is Ruff compliant (see ruff.toml).
• Your code is python 3.13 compliant.
• If this is a new feature and not a bug fix, you've included the proper documentation in the docs.
• Model changes must include the necessary migrations in the dojo/db_migrations folder.
• Add applicable tests to the unit tests.
• Add the proper label to categorize your PR.