[chore]: update readme (browserbase#1971)

<!-- This is an auto-generated description by cubic. -->
## Summary by cubic
Switched badge SVGs to green `#00C851` for Discord and License (light
and dark) and set their links to `display: inline-block` in
`packages/core/README.md` to keep badges aligned. Branding/readme-only;
no app code changes.

<sup>Written for commit 698eb0f.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1971">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

Author: seanmcguire12