Add some information about setting up a development environment

closes #1904 (closed)

What I did

• I created a folder 'development'
  • I split up the file dev.md into parts and moved it to this folder
    • index.md
    • authentication_authorization.md
  • I moved ap_extensions.md
  • I moved the API folder
  • I added the mrf info to docs/configuration/mrf.md
  • I created a new file setting_up_pleroma_dev.md

Extra

• I fixed some links
• When formatting all .ex and .exs files, it still reformats priv/repo/optional_migrations/rum_indexing/20190510135645_add_fts_index_to_objects_two.exs even though it doesn't fail the format check. I took the liberty of adding the reformatted file here as well.
• I changed the .gitignore to ignore /config/setup_db*.psql (see the wildcard) because I often use a variation of this file for the test-db which will now be ignored as well.