add binder badge to readme

[clmould]

Description

Adds a binder badge to the README - when clicked takes you to the binder site which renders PROCESS online, allowing the examples to be run there. Also added some instructions to the README and the docs to explain.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 1.51515% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.94%. Comparing base (0548fed) to head (b122737).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/io/plot_proc.py	1.51%	65 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3967      +/-   ##
==========================================
+ Coverage   45.89%   45.94%   +0.04%     
==========================================
  Files         123      123              
  Lines       29066    29791     +725     
==========================================
+ Hits        13340    13687     +347     
- Misses      15726    16104     +378     

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[timothy-nunn]

We need to ensure that the optional examples dependencies are also installed in the binder environment

[clmould]

I'm not sure why this was commented out on main ? I can comment back out/remove if needed

[clmould]

@timothy-nunn

[timothy-nunn]

@chris-ashe any idea?

[clmould]

Spoke with @chris-ashe and have commented it back out in this PR

[clmould]

I have removed the need for pdf2image by giving plot_proc an optional argument to produce png output instead of pdf, so the png files can be rendered in examples.ipynb by using %pip install pillow at the start of the notebook. pdf2image relies on poppler-utils which cannot be installed in the binder environment. Now the plot_proc cell runs in the binder:

This also solves #3981 :

[je-cook]

A few things here and there but overall looks good

[timothy-nunn]

Happy with the code changes, it works locally.

I need your help testing on binder before I approve