Base: Add man page for env

Author: https://github.com/tcl3
Commit: https://github.com/SerenityOS/serenity/commit/3de4cd0ba9
Pull-request: https://github.com/SerenityOS/serenity/pull/19630

Base/usr/share/man/man1/env.md

@@ -0,0 +1,19 @@
+## Name
+
+env - Execute a command with a modified environment
+
+## Synopsis
+
+```sh
+$ env [--ignore-environment] [--split-string S] [--unset name] [env/command...]
+```
+
+## Options
+
+* `-i`, `--ignore-environment`: Start with an empty environment
+* `-S S`, `--split-string S`: Process and split S into separate arguments; used to pass multiple arguments on shebang lines
+* `-u name`, `--unset name`: Remove variable from the environment
+
+## Arguments
+
+* `env/command`: Environment and commands