Fix docs build: point PyCall at Conda Python so SymPy loads

[ChrisRackauckas-Claude]

Problem

The documentation build on main is failing (the only red check on the default branch). After CI was centralized onto the reusable SciML documentation workflow (#219), the docs build can no longer build the SymPy/PyCall-based RK order conditions tutorial:

• On the runner image at the time of the original master failure, PyCall built against the system /usr/bin/python3 (which has no sympy), and using SymPy failed at runtime with ModuleNotFoundError: No module named 'sympy' / ref of NULL PyObject, terminating makedocs on :setup_block / :example_block.
• On the current ubuntu-latest image, PyCall now fails even earlier, during the workflow's Pkg.instantiate() install step, because that python3 (3.12) has no usable shared libpython: ERROR: Error building PyCall: Couldn't find libpython; check your PYTHON environment variable.

Root cause

The RK order conditions tutorial loads SymPy (via PyCall). PyCall, by default, builds against whatever system Python it finds on PATH. The repository's old (pre-#219) Documenter.yml set PYTHON: "" on every relevant step, forcing PyCall to use the private Conda.jl Python into which SymPy auto-installs sympy. When CI was centralized onto the reusable workflow, that PYTHON="" was dropped — and the reusable documentation.yml@v1 exposes no input/env hook to set it — so PyCall reverted to the (now unbuildable) system Python.

Fix

1. .github/workflows/Documenter.yml — make the Documentation job self-contained again and set PYTHON: "" on the install and build steps, so PyCall builds against the Conda Python. (This is the load-bearing CI fix; the reusable workflow cannot build PyCall/SymPy docs.)
2. docs/make.jl — also set ENV["PYTHON"] = "" and rebuild PyCall/SymPy at the top, so local docs builds work out-of-the-box regardless of the surrounding environment.

Local verification (Julia 1.12, the CI "1" channel)

• Reproduced the runtime failure by pointing PyCall at a Python venv without sympy: using SymPy failed with ModuleNotFoundError: No module named 'sympy' / ref of NULL PyObject.
• Observed the install-step libpython build failure in the PR CI run that used the bare reusable workflow.
• Confirmed PYTHON="" makes Pkg.build("PyCall") build against the Conda Python.
• Ran the full docs/make.jl to completion twice: exit code 0, all stages (Doctest, ExpandTemplates, CrossReferences, CheckDocument, RenderDocument, HTMLWriter) pass; the only warning is the expected "Skipping deployment" for a local run.

docs/make.jl is Runic-clean; Documenter.yml is valid YAML.

Please ignore until reviewed by @ChrisRackauckas