Documentation points to unavailable gcc version

## Expected Behavior
The documented gcc version, 7.6.0, is not available from conda-forge.

I was able to find `gcc_linux-64=7.5.0` through the conda-forge channel. With that TIGRE successfully built, so I recommend changing [the documentation](https://github.com/CERN/TIGRE/blob/master/Frontispiece/python_installation.md) to reflect that. The prior version I tried, 14.3.0, the version I got without specifying a version in my conda `environment.yml` file led to errors in the build process.

## Actual Behavior

## Code to reproduce the problem (If applicable)

This is the contents of the `environment.yml` file for the conda environment that enabled me to successfully install the python version of TIGRE.
```yml
name: base
channels:
- nvidia/label/cuda-12.8.1
- conda-forge
- defaults
dependencies:
- python>=3.7,<3.12
- cuda
- cudnn
- cuda-nvcc
- gcc_linux-64=7.5.0
- gxx_linux-64=7.5.0
- h5py
- matplotlib
- networkx
- numpy
- pyyaml
- scipy
- trimesh
- tqdm
- pip
- pip:
- python-fcl
```

## Specifications

- MATLAB/python version: 3.11.13
- OS: Ubuntu Linux via the following docker container: mambaorg/micromamba:ubuntu24.04
- CUDA version: 12.8.1

Open Frontispiece/python_installation.md and compare its documented GCC version with the conda-forge version described in the issue. Update the installation guidance to reflect the working gcc_linux-64 and gxx_linux-64 7.5.0 packages, then check that the documented environment remains consistent with the provided example.

Written by the indexing model from the issue text.