docs: update `02-jest.mdx` (#72061)
## 📚 Improving Documentation
Hello, I’ve updated the `02-jest.mdx` documentation with the following
improvements:
1. I added the comment, `"The content of this doc is shared between the
app and pages router."` This document is used by both the Pages Router
and App Router, and I found this comment in other files within the
`docs` directory.
2. ~~I introduced `<PagesOnly>` and `<AppOnly>` components. Previously,
when clicking links in the App Router page, it would navigate to the
Pages Router documentation. I’ve modified this so that links open in the
App Router documentation when viewed there.~~
3. Additional typo corrections:
1. Removed colons (`:`) from headings, as they aren’t used in other
docs.
2. Added missing double quotes (`"`).
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
