[docs] Added curl and SCP instruction for ipk file

[Orange-Tofu]

Encountered a situtaion were both the methods for installing ipk files was not working.
Added instructions for insatlling ipk file using curl and SCP. Making it easier for future users to install openwisp-config.

[nemesifier]

I am not a 100% sure about this, it may make more sense to ensure the latest versions of the packages in the official openwrt builds are up to date and recommend installing from there as it will be easier, and mention the latest builds as an option for more advanced users which do not need to be taught how to use unix via our documentation pages.

[Orange-Tofu]

I agree with your idea (@nemesisdesign ) of ensuring that latest packages are installed through OpenWrt. It is the easiest option for a beginner, but from my personal experience I believe that the official openwrt builds are not reliable as of now. And it will take some time to reach the desired level.
I suggest for the time being to atleast add a note on curl/SCP method and when latest versions can be provided through OpenWRT then the docs can be updated (if required).

[Orange-Tofu]

By note I mean, we can write, the config file can also be installed using curl and SCP, or we can just provide instruction for curl leaving SCP or vice-versa.