docs: trace/index.rst: add histogram-design.rst

Fix the following warning:

Documentation/trace/histogram-design.rst:
WARNING: document isn't included in any toctree

By adding 'histogram-design.rst' to the index.

Signed-off-by: Daniel W. S. Almeida <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: dwlsalmeida

Documentation/trace/index.rst

@@ -20,6 +20,7 @@ Linux Tracing Technologies
    events-msr
    mmiotrace
    histogram
+   histogram-design
    boottime-trace
    hwlat_detector
    intel_th