Add documentation for editing remote files on the server

[giordano]

Some points we may want to highlight:

• suggest to install the Remote - SSSH extension (although this can be made a dependency of Nisaba itself: https://github.com/oracc/nisaba/issues/10#issuecomment-1104975437)
• we can point to VS Code documentation about editing remote files for general information
• explain how to add new targets (but this is also covered by the Connect to a remote host section of VS docs)
• remind to install Nisaba also on the remote server instance. Not clear whether this is automatically updated or kept in-sync with the local one.