[synapse] Update guide / Add sliding sync

[taddydevil]

That escalted quickly. Did some minor changes as preparation for Matrix 2.0.

• Link to the Uberspace 6 guide is offline
• Updated the use of domains for a better preperation on uberspace with additional services like sliding sync or audio / video calls
• Add a hint for urllib3. On a new uberspace urllib3 v2 will be automatically be installed and does not support OpenSSL 1.0.2 anymore
• Removed some obsolete steps (e.g. public_baseurl uses as default the server-name)
• Added configuration of sliding-sync

I hope I didn't overengineered it ;)

closes #1625

[taddydevil]

@tillmannheigel Maybe you can review the changes. I saw the domains I used as a example often in other configuration or documentations, so I think this will be fine.

[nichtmax]

Just a quick heads up: We will review your PR in January, we're a bit understaffed in support and we all need a few days off for Christmas 🎄 🤶

[luto]

Great guide! I have nothing to add, thank you :)