docs: Explain how to get an instance of Slugger

[pawelgrimm]

Marked version: 4.2.12

Markdown flavor: n/a

Description

• Fixes Add documentation for Slugger #1484

Contributor

• Test(s) exist to ensure functionality and minimize regression (if no tests added, list tests covering this PR); or,
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

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• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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[UziTech]

Thanks 💯