Allow the base URL for hosting to be configured post-build

[mportiz08]

• Rationale: Adds support to allow a base URL to be configured after the app has already been built. In this way, the docc compiler can generate output that can be hosted at any base URL instead of just the root path at /.
• Risk: Medium.
• Risk Detail: There is added complexity to the build process to create a template that DocC can easily manipulate.
• Reward: High
• Reward Details: Allows DocC users to host their app at any given subpath of their website instead of just the root (this is needed for most GitHub Pages hosting scenarios).
• Original PR: Add static-hostable support, by allowing changing the baseUrl at runtime #24
• Issue: rdar://70509237
• Code Reviewed By: @mportiz08
• Testing Details: Unit tests were added and updated. The expected docc compiler workflows utilizing this new feature were manually tested along with normal existing usage.

[ethan-kusters]

@swift-ci please test