very minor adjustments to install docs

[katestange]

These are suggestions as I'm installing it afresh again. These are super minor good first issue type things.

1. in doc/install-ubuntu.md, suggest indicating you should choose a directory before cloning backscope, and that cloning will create a directory called backscope
2. in install-postgre.md the line db> \d could use a hashtag comment saying what this command does (like db> \q has)
3. in running-backscope.md one could include a link (assuming the default server URL) to http://127.0.0.1:5000/api/get_oeis_values/A000030/50 so that testing is as easy as a single click.

[katestange]

Also, if we have actually gotten rid of cypari2 for good, the docs don't reflect that.

[katestange]

1. in resetting-the-database.md, it might be good to reiterate that you probably need to use -U in the database commands like dropdb.
2. It might be nice to point out how to tell if a PR uses a new schema -- is it models.py?