INGRID pip (or similar open-source-free-to-obtain-package manager) install instructions

INGRID currently has installation instructions using `conda` from `anaconda`. Anaconda is not necessarily free-to-obtain, see https://www.anaconda.com/pricing. It would be very helpful to have user install instructions using `pip` or other free-to-obtain package managers.

I have been able to install INGRID `INGRID==2.1.0` using python 3.10.12
```
python3 -m pip install -r requirements.txt
python3 -m pip install wheel
python3 -m pip install . --no-build-isolation
```
and `pip` and the following `requirements.txt`

```
contourpy==1.3.2
cycler==0.12.1
docutils==0.23
fonttools==4.63.0
fortranformat==2.0.3
freeqdsk==0.5.2
kiwisolver==1.5.0
m2r2==0.3.4
matplotlib==3.10.9
mistune==0.8.4
mpmath==1.3.0
numpy==2.2.6
packaging==26.2
pillow==12.2.0
pyparsing==3.3.2
python-dateutil==2.9.0.post0
PyYAML==6.0.3
scipy==1.15.3
setuptools-git-version==1.0.3
six==1.17.0
sympy==1.14.0
tk==0.1.0
```

Find the current installation instructions that use conda from Anaconda, then review requirements.txt and the reported Python 3.10.12 installation commands. Document a pip or other free-to-obtain package-manager path and verify that it installs INGRID 2.1.0 successfully.

Written by the indexing model from the issue text.