Purge trailing whitespace in docs source files

[paultcochrane]

Trailing whitespace is forbidden in source code for this project
(implemented as part of the PEP8 checks), however the documentation
source files contained trailing whitespace. It seemed appropriate to
remove trailing whitespace in the documentation source files as well.

This PR contains only whitespace changes; there are no content changes. If you would like something done differently here, please let me know and I'll be more than hapy to update the PR and resubmit as necessary.

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

[tshirtman]

Thanks, just to be sure, did you check this had no impact on the rendering of the affected blocks? i’m pretty sure it shouldn’t, but i didn’t touch RST in some time and wouldn’t want such a change to have unforseen visible effects :).

[paultcochrane]

Yes, I checked that the rendering still looked ok. It wasn't that easy to check everything as thoroughly as I'd have liked, but anywhere that I thought the rendering might have changed, everything still looked ok. Would you like me to have another look to check for certain?

[tshirtman]

Sorry for lack of answer, thanks again!