Documentation cleanup

[astro-friedel]

Description

This PR is just a cleanup of the documentation. It was motivated by a recent PR where many small changes were made to the documentation files that were not really part of the PR (specifically trailing white spaces were automatically removed b y the IDE).

Specific changes that were made:

• remove all trailing white spaces
• wrap all lines at column 100 (seemed to be the most common word wrapping already in use), some lines still go beyond this limit due to long links, python code, and puncutuation
• added docs/stubs and docs/1-parsl-introduction.ipynb to .gitignore since they are generated
• minor spelling and grammer fixes

Changed Behaviour

There should be no outward changes introduced by this PR, just re-formatting of the documentation source files.

Fixes

Fixes # (issue)

Type of change

Choose which options apply, and delete the ones which do not apply.

• Update to human readable text: Documentation/error messages/comments
• Code maintenance/cleanup