Setting up Aleph for local development

[tillprochaska]

• The Getting Started section should mention that you need to start a worker to process documents and update the index.
• The Getting Started section should include instructions on how to execute the command to increase ES memory mappings inside of the Docker VM.
• We should explicitly mention the single-user mode, as it’s the easiest way to get up an running. We should probably mention it in the Getting Started section and also in the Configuration section.
• The Debugging section includes outdated instructions (alephdata/aleph#2317)