#3580604 "Documentation for site exports" - First pass including general site export instructions + detailed instructions for drush site export

Tim Hestenes Lehnenrequested to merge
issue/drupal_cms-3580604:3580604-documentation-for-site-exports into 2.x

First pass including general site export instructions + detailed instructions for drush site export.

Leaves out the additional page from the issue about deploying to hosting providers for now, in case it is controversial.

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