doc: rename file endings from .sgml to .xml

As they are now XML files. In PostgreSQL itself they remain with
the .sgml suffix for backwards compatibility, but that's not
important for us.

doc/legal.xml

@@ -0,0 +1,37 @@
+<!-- doc/legal.xml -->
+
+<date>2017</date>
+
+<copyright>
+ <year>2010-2019</year>
+ <holder>2ndQuadrant, Ltd.</holder>
+</copyright>
+
+<legalnotice id="legalnotice">
+ <title>Legal Notice</title>
+
+ <para>
+  <productname>repmgr</productname> is Copyright &copy; 2010-2019
+  by 2ndQuadrant, Ltd. All rights reserved.
+ </para>
+
+ <para>
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+ </para>
+ <para>
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+ </para>
+ <para>
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see
+   <ulink url="https://www.gnu.org/licenses/">https://www.gnu.org/licenses/</ulink>
+   to obtain one.
+ </para>
+
+</legalnotice>