docs: surface manual binary download on installation page (#3123)

Co-authored-by: fibibot <fibibot@users.noreply.github.com>
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

Author: fibibot

runtime/getting_started/installation.md

@@ -195,6 +195,8 @@ You can also build and install from source using
 cargo install deno --locked
 ```
 
+## Manual download
+
 Deno binaries can also be installed manually, by downloading a zip file at
 [github.com/denoland/deno/releases](https://github.com/denoland/deno/releases).
 These packages contain just a single executable file. You will have to set the