Fix placement of release features in nav

Some release/deployment nav items got a bit lost in the nav after a couple MRs that mixed things up a bit:

• !2235 (merged): This MR moved the environments page down to a new place in the Nav, but that page was the "parent" to several nested subpages about environments. These ended up getting accidentally nested into the variables page as a result.
• !2461 (merged): @axil noticed that these pages were now located under variables, which didn't make sense, but the fix placed them as top-level items in the main CI/CD Nav section, which still wasn't correct.

This MR moves these nav items down to the Deploy and release your application section, as nested items under Environments and deployments (though I've put Review Apps outside that group, below Releases).

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