Fix a few minor textual issues.
Give a brief outline of what the program will actually do.
Reformat and rewrite parts of the documentation to make it fit more our
currently preferred style (in particular, of documenting whole code blocks as
opposed to individual lines).
Adjust some text.
Adjust image size to ensure that pictures fit into a typical window.
Provide links to step-54.
Also build on step-53.
Build the OpenCASCADE wrapper documentation even if the library wasn't
configured with this external dependenct.
Break overlong lines.
Merge pull request #230 from bangerth/updates-to-step-54

Updates to step 54