Bump next and nextra-theme-docs

Bumps [next](https://github.com/vercel/next.js) and [nextra-theme-docs](https://github.com/shuding/nextra). These dependencies needed to be updated together.

Updates `next` from 12.1.0 to 15.1.0
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v12.1.0...v15.1.0)

Updates `nextra-theme-docs` from 1.2.6 to 3.2.5
- [Release notes](https://github.com/shuding/nextra/releases)
- [Commits](https://github.com/shuding/nextra/commits/[email protected])

---
updated-dependencies:
- dependency-name: next
  dependency-type: direct:production
- dependency-name: nextra-theme-docs
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>