Update doc: starknet-devnet is now part of asdf-vm community plugins

[ptisserand]

Usage related changes

Update asdf installation part since starknet-devnet is now part of asdf-vm community plugins

Development related changes

Checklist:

• Checked out the contribution guidelines
• Applied formatting - ./scripts/format.sh
• [] No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• No spelling errors - ./scripts/check_spelling.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
  • Once you make the PR reviewable, please avoid force-pushing
• Updated the docs if needed - ./website/README.md
• Linked the issues resolvable by this PR - linking info
• Updated the tests if needed; all passing - execution info

[FabijanC]

Thanks!