Add tutorial/user guide docs for opening files from dropbox/google drive and possibly streamline

[cmdcolin]

There are a couple non-obvious steps such as having to select track type

Couple things that could also be improved codewise

1. Pasting a dropbox could automatically select the dropbox button internet account probably
2. The list of adapter types should probably be sorted in the select drop down, hard to find which one you want
3. Could infer the filename without ?dl=0 to get an accurate adapter type guess without user interference on dropbox. On google drive it is not possible from the pasted string, but maybe can get filename from an api endpoint and do so? google drive url looks like e.g. https://drive.google.com/file/d/1PIvZCOJioK9eBL1Vuvfa4L_Fv9zTooHk/view?usp=sharing

[cmdcolin]

consolidated into the larger docs issue