docs: Update dev-guide

Show that ./ci/(un)install.sh must be run with root privileges and
temporarily remove the section about changing the Gunicorn permissions.

I propose removing the section about Gunicorn permissions since it doesn't reflect the current one-node installation state. While Gunicorn used to run as `www-data`, due to Archipelago it must now run as `root`. So, although the users should be advised to run Gunicorn as `www-data`, right now they cannot do so.

Once this restriction is lifted, we can advise the users on how to run Gunicorn with `www-data` permissions, while keeping their working folder permissions sane. I'd propose a solution (untested) like this:

```
setfacl -R -m g:rx:www-data ~/synnefo
setfacl -d -R -m g:rx:www-data ~/synnefo
```
which would utilize the Linux Access Control Lists to give read-execute permissions to `www-data` only.