Available Modules: Custom inserted text

[lbarraga]

This PR add a feature where custom MarkDown can be inserted into the available software MarkDown files.
The custom text files will be located at mkdocs/docs/HPC/only/gent/available_software/custom and must match the name of the module exactly.

For example:

• mkdocs/docs/HPC/only/gent/available_software
  • custom/ (new)
    • ...
    • Python.md
    • ...
  • detail/
    • ...
    • Python.md
    • ...

Where the content of custom/Python.md is:

We have a dedicated page on Python available [here](../../../../python.md).
For more info on python virtual environments, check out [this page](../../../../setting_up_python_virtual_environments.md)

After running scripts/available_software/available_software.py, the contents of detail/Python.md will be

Python
======

We have a dedicated page on Python available [here](../../../../python.md).
For more info on python virtual environments, check out [this page](../../../../setting_up_python_virtual_environments.md)

# Available modules


The overview below shows which Python installations are available per HPC-UGent Tier-2 cluster, ordered based on software version (new to old).
...

---

This PR already adds custom/ entries for AlphaFold, OpenFOAM, MATLAB and Python.
Files in the custom/ directory are excluded from search by mkdocs

[boegel]

@lbarraga Please also add some basic instructions in scripts/available_software/README on how custom bits of text can be injected in the software-specific pages

Alos, add mkdocs/docs/HPC/only/gent/available_software/README.md which explains which files are auto-generated, and which ones are not (custom/*.md)

[boegel]

@lbarraga A next step could be to auto-detect that there's a matching "software-specific best practices page"

This would involve:

• moving existing files like alphafold.md to software-specific-best-practices/AlphaFold.md (and adding a redirect in config/templates/hpc.template to avoid breaking the https://docs.hpc.ugent.be/alphafold URL);
• updating the available_software.py script to check whether there's a matching file under software-specific-best-practices, and if so automatically inject a line like "We have a dedicated page on Python available [here](http://localhost:8000/Linux/python/)."
• clean up the custom/*.md files being added here

Let's make that a separate PR, this one can be merged once missing README files are added