Add known issues and workarounds to install docs

[eloquence]

We won't be able to resolve all install issues immediately, so we should at least document known issues and workarounds for now:

1. "Recurse failed: none of the specified sources were found" (Install failure: "Recurse failed: none of the specified sources were found" securedrop-workstation#527). Workaround: Clear Salt cache, re-run

2. "Failed to return clean data" ("Failed to return clean data" in sd-log stage securedrop-workstation#519). Workaround: Re-run.

3. Transient network issues with some VMs (Transient network issues in guest VMs during install securedrop-workstation#514). Workaround: Re-run.

[eloquence]

@emkll observed an issue with dnf install not properly provisioning files to /srv/salt; not tracked yet, but it was resolved with dnf reinstall. Once we have more details we can document.