docs: add installation instructions

[thawn]

Description

This pull request adds installation instructions to the documentation

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases. <-N/A
• I've added relevant documentation.
• I've marked translation strings. <-N/A
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).
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Frontend

• I've followed the CSS/JS and React guidelines. <-N/A
• I've followed the web accessibility guidelines. <-N/A
• I've followed the user interface guidelines. <-N/A

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[thawn]

This is a start that describes how I got the plugin to run in a local test instance. It is likely incomplete, particularly with respect to a production installation.

Therefore, I kindly ask for a thorough review and feedback by a core developer.