Migrate CI pipeline docs to lstk - #879

Migrates the /aws/ci-pipelines section from the localstack CLI to lstk.

• All six CI provider pages (GitHub Actions, GitLab CI, CircleCI, CodeBuild, Travis CI, BitBucket) migrated to use lstk. In the end, I kept the BitBucket, CodeBuild, and Travis CI pages because each provided value that isn't immediately obvious from the best-practices page, or from the CI system's own docs. The setup-localstack GitHub Action does not support lstk, so was replaced by raw lstk commands.
• Added a new best-practices page, /aws/ci-pipelines/best-practices, to explain the CI process in more detail, providing generic code snippets to illustrate the high-level steps.
• Auth tokens are treated as mandatory throughout all the examples (there is no longer a "Community Edition"), and ephemeral-instance examples are removed (not supported by lstk).
• Only the GitHub Actions and CodeBuild examples were run against the real CI systems, mainly because I didn't have the time to create test accounts in the other systems. The others were migrated by inserting known-good code snippets, with the expectation that we'll properly test these (with automation) as a follow-up exercise.

Completes DEVX-992

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