Improve First Time Setup instructions

[jdaviescoates]

At present they just say:

On first visit, register yourself and sign in. You may want to disable registration after sign up to prevent misuse.
Federation is enabled by default. Check the federation docs if federated searches are not succeeding.

But, until you've made a user an admin, you can't disable registration, so they really need a link to https://docs.cloudron.io/apps/pixelfed/#admin too.

[girish]

@jdaviescoates added link, thanks.