Suggestion: document resource requirements

This issue has been migrated from [#6367](https://github.com/matrix-org/synapse/issues/6367).

---

**Description:**

Can some rough estimations of what CPU performance is needed for which kind of usage?

Example of usage
- room with 30 people with E2EE
- with most people from other instances
- bringing with IRC or Telegram, etc

Example of low end servers that are attractive to self host thing cheaply:
https://www.kimsufi.com/uk/servers.xml
Which limitations should we expect with CPUs such as these? :

https://ark.intel.com/content/www/us/en/ark/products/49489/intel-atom-processor-d425-512k-cache-1-80-ghz.html
https://ark.intel.com/content/www/us/en/ark/products/58917/intel-atom-processor-n2800-1m-cache-1-86-ghz.html