Add Nessus quick-start documentation

[rksharma-owg]

Description

The Nessus API landing page currently identifies the package as a technology preview but does not show how to instantiate the client or make a basic request. This adds a concise quick-start example based on the working example shared in #915. It demonstrates API-key authentication, retrieving server properties, and listing scans.

No dependencies are required.

Fixes #915

Type of change

• This change requires a documentation update

How Has This Been Tested?

• Strict documentation build: uv run --python 3.12 --isolated --group docs sphinx-build -W -b html docs /tmp/pytenable-docs-nessus
• Full local suite: 2,116 passed, 34 skipped, 6 xfailed, 3 xpassed; 95.50% coverage
• Ruff: uv run --python 3.12 --isolated --group dev ruff check tenable
• Fork preflight: Python 3.11, 3.12, 3.13, and 3.14 lint/tests/coverage plus code audit (run)

Test Configuration:

• Python Version(s) Tested: 3.11, 3.12, 3.13, 3.14 on Ubuntu; 3.12.13 on macOS
• Tenable.sc version (if necessary): N/A

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes