Make it clearer in the docs that vc2-bitstream-validator is also a reference decoder.

[mossblaser]

• Add a ToC section to https://bbc.github.io/vc2_conformance/cli/index.html with a section with a simple note about the decoder being the bitstream verifier
• Improve wording in the description of vc2-bitstream-validator on the same page to make its role as decoder more explicit.