afconvert: add page (#9683)

* Add `afconvert` page

* Apply review suggestions from:
- https://github.com/tldr-pages/tldr/pull/9683/files#r1059363364

pages/common/afconvert.md

@@ -0,0 +1,12 @@
+# afconvert
+
+> Convert between AFF and raw file formats.
+> More information: <https://manned.org/afconvert.1>.
+
+- Use a specific extension (default: `aff`):
+
+`afconvert -a {{extension}} {{path/to/input_file}} {{path/to/output_file1 path/to/output_file2 ...}}`
+
+- Use a specific compression level (default: `7`):
+
+`afconvert -X{{0..7}} {{path/to/input_file}} {{path/to/output_file1 path/to/output_file2 ...}}`