docs(getting-started): add 3.15 to nox example to match new test matrix

[r266-tech]

What does this PR do?

Adds the two new 3.15 rows that nox -s tests now prints, so the example output in docs/development/getting-started.rst matches the actual test matrix after #1190 added Python 3.15 (CPython and free-threaded) to pyproject.toml classifiers and noxfile.py.

Net diff: +2 / -0.

     nox > * tests-3.14: success
+    nox > * tests-3.15: success
     nox > * tests-3.13t: success
     nox > * tests-3.14t: success
+    nox > * tests-3.15t: success

Related Issue

Follow-up to #1190 (added 3.15 + 3.15t to test matrix, nox sessions, and classifiers). Same form as the merged #1184 nox example sync.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)

Changes Made

• docs/development/getting-started.rst: insert tests-3.15: success between tests-3.14 and tests-3.13t, and tests-3.15t: success after tests-3.14t, matching the order in noxfile.py (*PYTHON_VERSIONS, "3.13t", "3.14t", "3.15t", "pypy3.9", ...).

How to Test

Read-only docs sync; no test changes. The rendered output now mirrors what nox -s tests will print on a machine with 3.9–3.15 + 3.13t/3.14t/3.15t installed.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• N/A — docs-only change

[henryiii]

That is a long description for a two line change!