docs: clarify where to run the "Compiling the test cases" commands

[zzbnb4396]

What

The First Steps page currently shows the "Compiling the test cases" commands
(mkdir build && cd build && cmake .. && make check) right after telling new
users they can install pybind11 via pip install, with no hint that the
commands are only meaningful from the root of a source checkout of the
pybind11 repository, nor that the pip wheels do not contain the bundled
tests.

This adds three sentences to docs/basics.rst clarifying that:

• the commands must be run from the root of a source checkout of pybind11,
• they compile and run the test suite bundled in the repository's tests/
  directory,
• wheels installed via pip install pybind11 don't include those tests, so
  pip users should start from a git checkout instead (cross-linked to the
  installing page).

Why

Closes #3871. The reporter followed the page after a pip install and there
was no context about where to run the commands or where the test cases live;
the maintainer's reply in the thread invited exactly this kind of wording
(e.g. "starting with a git checkout").

How verified

• docutils parse of docs/basics.rst: no structural errors, baseline vs.
  modified compared (only expected Sphinx-role noise).
• git diff --check: clean.
• No other sections or version numbers touched; this is a pure docs change.

Suggested changelog entry:

• Docs: clarify that the "Compiling the test cases" commands on the First
  Steps page must be run from the root of a source checkout of pybind11,
  since pip wheels don't include the bundled test suite.

---

📚 Documentation preview 📚: https://pybind11--6170.org.readthedocs.build/