docs: Migrate Precompiles chapter to inline .sol API docs

[matevz]

• migrates https://docs.oasis.io/dapp/sapphire/precompiles to .sol files viewable on https://api.docs.oasis.io
• makes all natspec documentation consistent accross .sol files:
  • always use @notice to describe what the function does so the comment is included in the API reference
  • optionally extend the the description above with implementation details using the @dev directive
• workaround for Doc generator strips all white spaces from comments foundry-rs/foundry#6619:
  • moves the initial example from Sapphire.sol to README.md because forge doc removes indentation
  • flattens two-level bullets and replaces it with subsubsubsubheaders instead
  • introduce subsubsubheaders when paragraphs really needed
• adds per-page TOC (requires cargo install mdbook-pagetoc), updated CI, README and packages.json accordingly

For preview

cd contracts
cargo install mdbook-pagetoc
pnpm doc

And open contracts/sol/sapphire-contracts/book/index.html in a browser.