Document time

This is a bit barebones, but `time` isn't super complicated to use and
it should explain the basics.

Fixes #6442

[ci skip]

sphinx_doc_src/cmds/time.rst

@@ -0,0 +1,39 @@
+.. _cmd-time:
+
+time - measure how long a command or block takes
+================================================
+
+Synopsis
+--------
+
+::
+
+   time COMMAND
+
+Description
+-----------
+
+``time`` causes fish to measure how long a command takes and print the results afterwards. The command can be a simple fish command or a block. The results can not currently be redirected.
+
+For checking timing after a command has completed, check :ref:`$CMD_DURATION <variables-special>`.
+
+Example
+-------
+
+(for obvious reasons exact results will vary on your system)
+
+::
+
+   >_ time sleep 1s
+   
+   ________________________________________________________
+   Executed in    1,01 secs   fish           external
+      usr time    2,32 millis    0,00 micros    2,32 millis
+      sys time    0,88 millis  877,00 micros    0,00 millis
+
+   >_ time for i in 1 2 3; sleep 1s; end
+
+   ________________________________________________________
+   Executed in    3,01 secs   fish           external
+      usr time    9,16 millis    2,94 millis    6,23 millis
+      sys time    0,23 millis    0,00 millis    0,23 millis