Allow wrapping for ARM table content

RTD style default never wraps <th> and <td> elements and that just does
not work for real sentences or any other long lines.

We can reconsider styling some tables separately, but at the moment we
do not have use for tables with long but unwrappable lines so it's
easier to allow wrapping globally.

(cherry picked from commit a5dd98ac1b)

doc/arm/Makefile.am

@@ -61,6 +61,7 @@ EXTRA_DIST =					\
 	troubleshooting.inc.rst			\
 	tsig.inc.rst				\
 	zones.inc.rst				\
+	_static/custom.css			\
 	../dnssec-guide				\
 	../misc/acl.grammar.rst			\
 	../misc/controls.grammar.rst		\

doc/arm/_static/custom.css

@@ -0,0 +1,17 @@
+/*
+Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+
+SPDX-License-Identifier: MPL-2.0
+
+This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0.  If a copy of the MPL was not distributed with this
+file, you can obtain one at https://mozilla.org/MPL/2.0/.
+
+See the COPYRIGHT file distributed with this work for additional
+information regarding copyright ownership.
+*/
+
+/* make table content wrappable */
+.wy-table-responsive table td {
+    white-space:normal;
+}

doc/arm/conf.py

@@ -154,6 +154,10 @@ master_doc = 'index'
 # a list of builtin themes.
 #
 html_theme = 'sphinx_rtd_theme'
+html_static_path = ['_static']
+html_css_files = [
+    'custom.css'
+]
 
 # -- Options for EPUB output -------------------------------------------------