docs: clarify that prebuilt downloads are standalone executables (#3632)

State explicitly in the README and download page that the
platform-specific prebuilt files are standalone executables, not
archives. Include the rename and executable-permission step for macOS
and Linux in the README.

Addresses #2956. Source archives are not described as executables.

Validation: the official `docs/build_website.py` completed and the
generated download page contains the new guidance. Used a Python venv
with the site's required rendering packages; restored the Windows
checkout's manual symlink in a temporary build copy only. jq itself was
not built or tested for this documentation-only change.

Co-authored-by: Hugohong258 <282228220+Hugohong258@users.noreply.github.com>

README.md

@@ -12,6 +12,8 @@
 ### Prebuilt Binaries
 
 Download the latest releases from the [GitHub release page](https://github.com/jqlang/jq/releases).
+The platform-specific assets are standalone executables, not archives. On macOS and Linux,
+rename the downloaded file to `jq` and make it executable with `chmod +x jq`.
 
 ### Docker Image