[5.7] Unrevert "Transform DocC archives for static hosting by default"

[ethan-kusters]

Re-nominating #121 for release/5.7 after this change had to be reverted due to an integration test failure. This change was originially landed on release/5.7 with #136 and reverted on release/5.7 with #147.

Original nomination is as follows:

• Rationale: Updates docc convert to transform for static hosting by default. Forums discussion here.
• Risk: Low.
• Risk Detail: This enables a feature that has already shipped in Swift 5.6 and is being widely used by default.
• Reward: High
• Reward Details: This change would mean that, by default, a DocC archive can essentially be copied into the root of most server hosts and "just work".
• Original PR: Transform DocC archives for static hosting by default #121
• Issue: rdar://91173450
• Code Reviewed By: @franklinsch and @Kyle-Ye
• Testing Details: Existing tests continue to pass. A test that confirms that static hosting transformation is enabled by default when no flags are passed to docc convert has been added.

Since reverting this, both the integration test and docc have been updated to avoid the failure we saw when originally landing this:

• [5.7] Remove confusing error when no docs are produced #149
• [5.7] Fix SwiftDocC integration test swift-integration-tests#102

[ethan-kusters]

@swift-ci please test

[ethan-kusters]

@swift-ci please test

[ethan-kusters]

Swift integration tests are passing with this PR here: swiftlang/swift#42440.