Docs/add pytest approx mentions

[tanuj-rai]

Closes #13662

This PR adds documentation references to pytest.approx as suggested in #13662.

Changes

• Added a note about pytest.approx in getting-started.rst
• Added a dedicated "Comparing floating point numbers" section in how-to/assert.rst

Here is a quick checklist that should be present in PRs.

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.

[tanuj-rai]

Thanks, @nicoddemus! I’ve removed the .. note:: directive and replaced :ref:\approxwith:func:pytest.approx`` as suggested. Please let me know if anything else should be adjusted.

[nicoddemus]

Looks great, thanks!