Overhaul of "doc" build target for generating policy script documentation.

It's now all implemented in CMake scripting.

The generation of reST docs is now a distinct target, "restdoc", while
the target to generate HTML docs, "doc", depends on "restdoc".  reST doc
generation supports incremental builds (documentation for a given policy
script is only regenerated when it is out of date), but HTML doc generation
via ``make doc`` is not incremental (Sphinx always starts with fresh input).

Building the "restdoc" target is now covered by a btest to ensure all
policy scripts are parse-able when Bro is in "doc mode".

Generated reST docs should now support "@load"ing from subdirectories.  e.g.
"@load foo/baz" and "@load bar/baz" will now generate the right xref links.

doc/scripts/source/index.rst

@@ -11,11 +11,11 @@ Contents:
 
    common
    builtins
-   policy/default
-   policy/user
-   policy/bifs
-   policy/internal
    policy/index
+   default
+   bifs
+   user
+   internal
 
 Indices and tables
 ==================

doc/scripts/source/policy/index.rst

@@ -1,10 +1,6 @@
 Index of All Policy Script Documentation
 ========================================
 
-Contents:
-
 .. toctree::
    :maxdepth: 1
-   :glob:
 
-   *