Update docs and add doctest loading mechanism

[althonos]

Type of changes

• Documentation / docstrings
• Tests

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

This PR addresses some documentation issues. I also added a unittest loader so that the documentation examples are now tested with the other unit tests. This will prevent the documentation from having out-of-date examples.

• Closes open_fs(".", writeable=False) returns an FS object which can write #441
• Closes Example for TempFS looks wrong #423
• Closes WriteTarFs temp_fs arg type error #410
• Closes Documentation for copy throwing FileExpected exception missing #300
• Closes Standardize and enforce behaviour of FS.removetree("/") #471
• Closes Deprecate FS.getbasic as it is redundant with current FS.getinfo behaviour #469

[coveralls]

Coverage decreased (-0.01%) to 95.355% when pulling 41bbe3c on update-docs into e6f0c8f on master.