DOCSP-4384 Add analytics snippet to jazzy docs

[cbush]

• As in other sdks, when the argument "analytics" is passed to generate_docs.sh, then the analytics snippet will be added.
• Jazzy base theme "fullwidth" is copied to contrib/docs-theme/ and the analytics snippet is added there.
• Local builds of docs (i.e. without passing "analytics" argument) will produce docs without analytics, using the standard fullwidth theme.

Staged: https://docs-mongodbcom-staging.corp.mongodb.com/stitch/bush/docsp-4383-analytics/sdk/swift/index.html

Note: to verify,

• view source and see analytics snippet in the header, something like

  <script>
      !function(){var analytics=window.analytics=window.analytics ........

• go to https://app.segment.com/mongodb/sources/docs/debugger and see incoming analytics from staging.

[adamchel]

and evergreen is already broken until they install Xcode 10.2 so it should be okay to merge

[cbush]

Linting passed, evg otherwise broken until Xcode 10.2 installed. Merging.