Adds a white background to the Infrastructure diagram

[bibliotechy]

❤️ Thank you for your contribution!

Description

Adds a white background to the infrastructure diagram so it is legible in dark mode

Fixes #931

Checklist

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

• [x ] I'm aware of the code of conduct.
• [x ] I've created logical separate commits and followed the commit message format.
• [ x] I've targeted the master branch.
• [x ] If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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[fenekku]

Ha! Thanks for that. Must admit I mostly read it in light mode indeed.