docs: Add `CONTRIBUTING.md`

[juliaogris]

Abandoned initial PR: https://github.com/evylang/evy/pull/285

Also add ISSUE template probably under /.github directory. ISSUE template should state that evy repo issues are only for bugs. Feature requests got to evylang/todo after discord discussion.

Explain that this repo aims to be an educational repo and therefore tries to set standards high. An "opinionated" repo.

Suggest filing bug-report.

Explain project management on evylang/todo.

Feature proposals should go to discord first.

explain development workflow, emphasis on good commits and git-scripts (link to think about the reviewer).

Add that contributions in form of Evy programs for the gallery are much welcome.

[Rossiar]

Some ideas for CONTRIBUTING.md content based on my first PR, mainly sourced from @juliaogris' initial feedback: https://github.com/evylang/evy/pull/249#pullrequestreview-1837737941

• commit style
• comment style (wrapping comments at 72 chars)
• file naming style (no underscore unless _test or _arch)
• testing methodology
• merge script instructions (although the repo probably shouldn't allow just anyone to do this)