Update 4.1 highlights [ci skip]
Clearified what Spring actually does [ci skip]
Variants is probably too generic? [ci skip]
Added a section for mailer previews in the release notes [ci skip]
Re-arranged things to match the highlights at the top [ci skip]
Put Spring before the code-level features to group things a little bit better [ci skip]
Upgrading from what? [ci skip]
Added secrets.yml to the release notes and upgrade guide [ci skip]
Added link for upgrading existing applications to use spring in the release notes [ci skip]
Added release notes for concerning [ci skip]
Removed line numbers from the doc links in the release notes, because they change too often [ci skip]
Moved enum release notes to where it should be [ci skip]