DOC: Add open-source contribution docs and issue templates

[kishorereddy]

Overview

Adds the docs a project needs to take outside contributions in a structured way: a
CONTRIBUTING.md, a CHANGELOG.md, GitHub issue templates, and an updated PR template.

Ticket(s)

N/A

Links(s)

N/A

Example(s)

• Opening a new issue now offers a Bug report or Feature request template instead of a
  blank box.
• The PR template now opens with a Related discussion or issue section, matching the
  "discuss before you code" flow described in CONTRIBUTING.md.

Dependencies

None.

Design

1. CONTRIBUTING.md: explains the project is early and not open to general PRs yet, what kind
   of input is welcome right now (bug reports, design feedback, questions), and the bar for
   hosting an official language port (a real sum type with compiler-enforced exhaustiveness).
   Includes a language candidate table (TypeScript, Scala, Rust, others) as a starting reference.
2. CHANGELOG.md: started fresh, covering 1.0.0 and the 1.0.1 OMITTED wording fix,
   using the standard Keep a Changelog format.
3. Issue templates: added bug_report.md and feature_request.md under
   .github/ISSUE_TEMPLATE/, both scoped to the same simple, structured format.
4. PR template: reordered to lead with a link to the originating Discussion/Issue, and moved
   Notes to the end so it reads as a closing thought rather than a mid-template detour.
5. README link: fixed the Issues link from github.com/slatekit/... to
   github.com/kiitdev/..., ahead of a planned GitHub org rename to match the kiit.dev domain.
   The live CI badge and build.gradle.kts's SCM metadata were deliberately left as slatekit
   for now, since those would actually break today, they'll get updated once the org rename
   itself happens.

Notes

This mirrors the same docs already added to kiit-result, keeping both repos' contribution flow
consistent.

Pending

The slatekit reference in the CI badge and build.gradle.kts's scm block is intentionally
unchanged, tracked as follow-up once the GitHub org is actually renamed.

Tests

None, docs only.