docs: add CONTRIBUTING.md

[mbelyayev]

What

Adds CONTRIBUTING.md with more detailed information on how to get started, fills gaps on how to clone in-of or out-of a test vault, with HMR an option for cloning into the test vault via the hot-reload community plugin. Based on my own experience the current section in the README was lacking a bit.

How to check

The code blocks in both setup scenarios for a fresh test vault execute and the CI pnpm commands pass.

Checklist

• pnpm check, pnpm check:submission and pnpm test pass
• One change per PR
• CHANGELOG.md has an entry under Unreleased if users will notice the change
• The styleguide gallery is updated if a UI component changed

[mbelyayev]

Looking to aid contribution! I'm looking for a successor to obsidian-projects and this project looks promising! Especially interested in #197 and #200. Thank you.