docs: address issue #23757

This change was automatically generated by the documentation agent team
in response to issue #23757.

🤖 Generated with cagent

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

Author: github-actions[bot]

_vale/config/vocabularies/Docker/accept.txt

@@ -1,5 +1,7 @@
 (?i)[A-Z]{2,}'?s
 jq
+zstd
+Zstandard
 untracked
 ripgrep
 exfiltration

content/manuals/build/exporters/_index.md

@@ -252,6 +252,22 @@ the previous compression algorithm.
 > The `gzip` and `estargz` compression methods use the [`compress/gzip` package](https://pkg.go.dev/compress/gzip),
 > while `zstd` uses the [`github.com/klauspost/compress/zstd` package](https://github.com/klauspost/compress/tree/master/zstd).
 
+#### zstd compression levels
+
+When you specify `compression=zstd`, the `compression-level` parameter accepts
+values from 0 to 22. BuildKit maps these values to four internal compression
+levels:
+
+| compression-level | Internal level | Approximate zstd level | Description                           |
+| ----------------- | -------------- | ---------------------- | ------------------------------------- |
+| 0-2               | Fastest        | ~1                     | Fastest compression, larger file size |
+| 3-6 (default)     | Default        | ~3                     | Balanced compression and speed        |
+| 7-8               | Better         | ~7                     | Better compression, slower            |
+| 9-22              | Best           | ~11                    | Best compression, slowest             |
+
+For example, setting `compression-level=5` and `compression-level=6` produces
+the same compression output, since both map to the "Default" internal level.
+
 ### OCI media types
 
 The `image`, `registry`, `oci` and `docker` exporters create container images.