docs: histogram.txt: convert it to ReST file format

Despite being mentioned at Documentation/trace/ftrace.rst as a
rst file, this file was still a text one, with several issues.

Convert it to ReST and add it to the trace index:
- Mark the document title as such;
- Identify and indent the literal blocks;
- Use the proper markups for table.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/events.rst

@@ -524,4 +524,4 @@ The following commands are supported:
   totals derived from one or more trace event format fields and/or
   event counts (hitcount).
 
-  See Documentation/trace/histogram.txt for details and examples.
+  See Documentation/trace/histogram.rst for details and examples.

Documentation/trace/index.rst

@@ -18,6 +18,7 @@ Linux Tracing Technologies
    events-nmi
    events-msr
    mmiotrace
+   histogram
    hwlat_detector
    intel_th
    stm

kernel/trace/Kconfig

@@ -605,7 +605,7 @@ config HIST_TRIGGERS
 	  Inter-event tracing of quantities such as latencies is also
 	  supported using hist triggers under this option.
 
-	  See Documentation/trace/histogram.txt.
+	  See Documentation/trace/histogram.rst.
 	  If in doubt, say N.
 
 config MMIOTRACE_TEST