Some small details in doc (e.g., SCPE_v0.2.png and link error to PyPSA)

[gincrement]

Closes # (if applicable).

Changes proposed in this Pull Request

• missing SCPE_v02.png
• add highs installation not
• adjust link to PyPSA examples

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[ekatef]

Hello @gincrement and many thanks for the fixes!
I very much like the polished README and looking forward to merge the changes.

A couple of technical comments:

1. May it be possible to replace the image file with a link to it? We used to commit binary files into the repo previously but it had lead to the need to hard-reset the main at some point. Starting from that moment we aim to avoid committing images into the code repo 🙂

Can we probably use a link to an image uploaded as a part of the paper on the sector-coupled model? E.g. that is a picture in high resolution.

2. Completely support advocating open solvers, and adding installation instructions for HiGHS is a very good idea. But I see a risk linked with modifying the environment, especially for the first-time users.

We have observed a few cases when installing some additional package lead to dependency conflicts and weird errors, which are quite difficult to resolve. May it be a good idea to move the explanations on HiGHS to pypsa-earth/doc/installation.rst, replacing this very generic instruction we have there now? My feeling is that it could be a more failure-proof design as first-time users are less likely to copy-paste instructions from read-the-docs, while for README it's quite a usual pattern. What do you think?