Review and update minimum RAM requirements in docs
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Issue body
### Bug Description
All documents that cover the minimum requirements set the minimum memory at 4GB.
We've received recent feedback from two sources that the tutorials are extremely slow at even 8GB because of Multipass.
We should determine what a more realistic minimum is and update all pages with it.
### To Reproduce
-
### Environment
Machines running Linux.
### charmcraft.yaml
```yaml
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```
### Relevant log output
```shell
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```
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