checks: add explanatory images

[carlinmack]

❤️ Thank you for your contribution!

Description

Please describe briefly your pull request.

I'm currently working on adding asynchronous checks, and re-learning how this fits together made me realise we could use an image or two.

I think that we (me + Maira) will probably develop this documentation a fair bit, but I think this is a good contribution which could be backported to the live 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 targeted the master branch.
• 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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