Docs: add link to spartan-parallel on hackmd

[kunxian-xia]

This PR

• renames the documentations directory to docs as it's more concise.
• pastes the link to copies of our docs on HackMD since math is not rendered well on GitHub.

[matthiasgoergens]

Thanks. We can consider removing the math markdown from the repository, so we don't have to keep two copies in sync?

[kunxian-xia]

The copies on HackMD is currently under my personal account https://hackmd.io/@ceno-zkvm which is subject to changes. Therefore it's better to keep the original docs in the repo.