Fixed #20500 - Updated flatpages URLconf example to work with APPEND_SLASH.

Thanks josh.23.french@.
Fixed #20462 - Fixed sqlite regex lookups for null values and non-string fields.
Added docs subdir to gitignore

To be able to build translated documentation, docs/locale should
be a symbolic link to the django-docs-translations/translations
directory.
Revert "Fixed #20462 - Fixed sqlite regex lookups for null values and non-string fields."

This reverts commit 64041f0e6e7a773574d7cde978d16305200004ea.

lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.
fixes #20486, file_move_safe will now check existence of destination if allow_overwrite is False