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/index.rst

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