Base: Add man page for purge(8)

Base/usr/share/man/man8/purge.md

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+## Name
+
+purge - release memory
+
+## Synopsis
+
+```**sh
+$ purge [options]
+```
+
+## Description
+
+This program instructs the kernel to release memory that can
+be released without interacting with its userspace owner(s).
+
+## Options
+
+* `-c`: Release all clean inode-backed memory.
+* `-v`: Release all purgeable memory currently marked volatile.
+
+If no options are specified, all possible memory is released.
+
+## Examples
+
+```sh
+$ purge
+Purged page count: 744
+```