docs: add an intro paragraph to the glossary (#98654)
The glossary page goes straight from frontmatter to the first entry, `#
A` — the word "glossary" appears nowhere in its body.
We index docs for search from the rendered content, and the page title
is stored as non-searchable metadata, so searching "glossary" on
nextjs.org does not return the glossary.
This adds a two-sentence intro, which gives the page a body chunk
carrying its own name.
```mdx
The Next.js documentation uses a recurring set of terms for routing, rendering, and caching. This glossary defines them.
```
Also reads better than a page that opens directly to content.
---------
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Aurora Scharff <66901228+aurorascharff@users.noreply.github.com>
