Update PROTEUS schematic

[stuitje]

Description

This PR updates the proteus-framework.md page with the new PROTEUS schematic via JSdelivr. For the light+dark switch and correct size, a new stylesheet is added; this is extra.css. The old extra.css has been renamed to proteus_theme.css since this configures the general theme of the website. footnotes.css has been updated to remove redundant lines.

It also updates the Atmodeller+FastChem links, and updats the README with the current installation instructions (changed since last PR; now a get_socrates.sh script is used).

Validation of changes

Built the site locally on safari, firefox and chrome.

Checklist

• I have followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have checked that the tests still pass on my computer
• I have updated the docs, as appropriate
• I have added tests for these changes, as appropriate
• I have checked that all dependencies have been updated, as required

[nichollsh]

Looks great. Thank you, also, for correcting the outdated information in the installation instructions.