Update docs and use rocksdb as feature

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Separate rocksdb usage into feature
  • No need to have to compile all of that when it's just used inside the forest crate, it was starting to get annoying
• Update docs to indicate for error: failed to run custom build command for neptune-triton v0.2.2 #420
  • If anyone has something useful to link for this please comment, it is an OS specific installation and no good central resource that's useful that I could find
• Update PR template
  • Has been slightly bugging me for awhile but way it is people always put the issue on the next line and it doesn't connect to auto close issue

Reference issue to close (if applicable)

Closes #420

Other information and links