Update README to be more in line with our version of the UI

[fhennig]

The README is missing some info about the project structure.

Link to new README: https://github.com/stackabletech/documentation-ui/tree/docs/building

What about the LICENSE and copyright notice, do these still apply?

[stackable-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ fhennig
❌ Felix Hennig

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[lfrancke]

CLA bot is wrong here. @fhennig was an employee at the time he created this PR and I am just bringing it over the finishing line.