Prepare documentation for publication

[weierophinney]

• Renamed existing files to remove zend.math. prefix.
• Split introduction into:
  • intro.md, containing only an overview of functionality provided.
  • rand.md, covering the Zend\Math\Rand functionality.
  • big-integer.md, covering the Zend\Math\BigInteger functionality.
• Edited for formatting and content.
• Converted from bookdown to mkdocs.
• Added documentation build automation.

Preview at: https://zendframework.github.io/zend-math/