docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/source/about.rst
• docs/source/ddf.rst
• docs/source/index.rst
• docs/source/more.rst
• docs/source/overview.rst
• docs/source/settings.rst

Fixes:

• Should read pollutes rather than polutes.
• Should read maintain rather than mantain.
• Should read satisfactory rather than satisfatory.
• Should read relationships rather than relatioships.
• Should read purposes rather than pourposes.
• Should read indefinitely rather than indefinetely.
• Should read explicitly rather than explicitily.
• Should read example rather than exampel.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md