docs: getting-started guide should not recommend primary entry-point

[devversion]

Currently it looks like the getting-started guide recommends using the
primary entry-point for importing the slider module. Instead we should
use the secondary entry-point to align the guide with the upcoming V9
breaking change.

Also adds a new line to the end of the file.

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: bazel_build" is failing
     status "ci/circleci: ivy_test" is failing
     status "ci/circleci: tests_browserstack" is failing
     status "ci/circleci: tests_local_browsers" is failing
     status "ci/circleci: tests_saucelabs" is failing

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