Clean up stale documentation and references

[antonwolfy]

This PR cleans up stale documentation and references, one commit per change:

1. Quick start guide. The testing section suggested pytest -s tests, but there is no top-level tests directory; it now gives python -m pytest dpnp/tests for the source tree and python -m pytest --pyargs dpnp.tests for an installed package. The scikit-build recipes now pass --no-build-isolation to pip install -e ., as scripts/build_locally.py does, so pip doesn't rebuild the package in an isolated environment with build dependencies from PyPI. The guide now says the driver script works on Linux and Windows, and a code-block that didn't render because it had no blank line before its content is fixed.
2. API reference toctree. doc/reference/index.rst listed 14 pages that don't exist (special, sparse, ndimage, generic, memory, cuda, memoize, kernel, optimize, interoperability, testing, prof, environment, misc); they are removed. doc/reference/polynomials.rst, a placeholder whose entries are all commented out and which routines.rst leaves out on purpose, is marked :orphan:. Both produced Sphinx warnings.
3. Stale files and references.
   • .github/workflows/Windows-IntelLLVM_3.22.cmake is removed: nothing uses it, and conda-recipe/bld.bat applies only the 3.26 copy.
   • doc/docstring_template.py described a fallback to NumPy and an array printing format that no longer exist; it now shows the docstring layout dpnp uses.
   • The 110 dpctl.tensor references left in dpnp/tensor docstrings and comments now say dpnp.tensor. Every referenced name exists in dpnp.tensor; on the same lines, :module: is fixed to :mod: and a misspelled usm_ndarrays to usm_ndarray.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/3106/index.html

[coveralls]

No base build to compare — docs-cleanup into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev14=np2py314h8d9cdd5_7 ran successfully.
Passed: 1376
Failed: 0
Skipped: 6