Improved installation and usage docs

Description

Installation and usage documentation needs improvements and cleansing.

Tasks

• Certificates page:
  • The new onionspray-certs.conf file.
• Update the installation docs, including:
  • Reference to the usage docs (using.md).
• Using Onionspray (using.md):
  • Configuration options (pointing to the examples/ folder, the list of settings and default values (#44)).
  • Reconfiguring a project.
  • Needs to ensure Onionspray starts at boot, in three approaches:
    • Cron approach, using a /etc/cron.daily/onionspray-service file.
    • Trying with the command line.
    • Setting up a Systemd service, like in the Onionspray role.
  • Log rotation (so it does not fill the disk):
    • Reference implementation for Ansible.
• Mining:
  • Move mining software references to Onionmine docs (Alternatives page).
  • Point that Onionmine has an export-to-onionspray command.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)