Documentation

[dnut]

Addresses #43

Added docs to explain how to get up and running with Sig.

• Added new content to readme.md:
  • Links to blog posts and other docs in the repo
  • Setting up a development environment
  • Building Sig
  • Running Sig: subcommands (e.g. gossip) and CLI options
  • Running the tests and benchmarks
• Moved API documentation into docs/api.md and linked to it from the "Import Sig" section of the readme.
• Fixed incorrect license and zig version icons in the readme header
• Fixed inconsistent sig version in build.zig.zon (to be consistent with cmd.zig)

See the updated readme in a realistic looking context: https://github.com/dnut/sig/tree/docs-improvements

Review Prompts:

1. Should we just publish the readme to github.io as is, or should we create another page to publish there?
2. Do the section structure and section headings make sense?
3. Should I move any content from the readme to CONTRIBUTING.md? For example, environment setup or running tests. I feel it is more discoverable in the readme.
4. Should I change the header of the readme, for example adding a logo?