docs: fix the dead Discord invite and align on #start-here

[iayanpahwa]

Summary

CONTRIBUTING.md and the new-issue chooser linked gQyXjVBF, which no longer resolves. README.md and pyproject.toml used a live but different invite. All five references now use https://discord.gg/QFTD6tr9zu, which opens in #start-here and matches www.humanbound.ai.

Type of change

• Documentation only

Checklist

• CHANGELOG.md updated under [Unreleased]
• All commits are signed off (git commit -s)

One caveat: the pyproject.toml link is what PyPI renders in the project sidebar, so it only updates there on the next release.

Linked issue(s)

Closes #19

Companion PRs

The same change in the other four repos:

• docs: replace the dead Discord invite in the org defaults .github#2
• docs: use the #start-here Discord invite humanbound#133
• docs: replace the dead Discord invite plugins#8
• docs: use the #start-here Discord invite actions#10

Cross-repo context is in humanbound/humanbound#132.

[iayanpahwa]

The Lint + type-check failure here predates this branch.

ruff format --check . also formats Python blocks inside markdown, and it trips on the fw.evaluate([...]) snippet at README.md:99. This PR does not touch that snippet; the only README lines it changes are the two Discord URLs at 27 and 153.

Same failure on main with nothing applied:

$ git worktree add /tmp/fwmain upstream/main && cd /tmp/fwmain
$ ruff format --check README.md
1 file would be reformatted
  --> README.md:99:22

Happy to reformat that snippet in a separate PR if you want the branch green, but it seemed out of scope here.