DOC: improve documentation on install app of specific commit without making release

[unkcpz]

I encounter this when I help people in MarketPlace and they complaint that they need to wait 30 min to see aiidalab registry update and can have the new version installed from the app store.

We want to document weel that app developers or if user want to install the very latest version that are not being released by the app developer yet to test the latest fixes. The code can be something like below is also possible.

$ aiidalab install aiidalab-mfa-cscs@git+https://github.com/aiidalab/aiidalab-mfa-cscs@main
$ aiidalab install aiidalab-mfa-cscs@git+https://github.com/aiidalab/aiidalab-mfa-cscs@7d7ce4e

Despite it can be seen when executing aiidalab install --help we need to write somewhere in the doc or maybe in the app manager to giving a hint on how to do the installation manually.