Clean up text and links in screenshots page of user docs.
Tweak aesthetics of plots in screenshots page.

These changes also cleanup the code a bit, as described by MEP 12.
Add streamplot function to screenshots doc
Merge remote-tracking branch 'tonysyu/screenshots-update' into doc-fixes-1-3

Conflicts:
	doc/users/screenshots.rst
Fixes a number of problems in the doc build.
double backticks
Remove long URL
Merge pull request #2041 from mdboom/doc-fixes-1-3

Fix a number of issues in the doc build