Docs: make external packages optional

It was noted in #2155 that when packaging Freeciv21, sphinx_rtd_theme and
sphinx_last_updated_by_git were needed at build time to create the man pages.
The rtd theme is obviously only needed for HTML output and last_updated_by_git
doesn't seem to be used with the man pages. Make them both optional by checking
if they can be imported when evaluating the config.

This will allow package maintainers to ship man pages even when one of the two
packages are missing from their distribution.

docs/conf.py

@@ -15,7 +15,17 @@
 # import os
 # import sys
 # sys.path.insert(0, os.path.abspath('.'))
-import sphinx_rtd_theme
+try:
+    import sphinx_rtd_theme
+    has_theme = True
+except ImportError:
+    has_theme = False
+
+try:
+    import sphinx_last_updated_by_git
+    has_updated_by_git = True
+except ImportError:
+    has_updated_by_git = False
 
 
 # -- Project information -----------------------------------------------------
@@ -48,9 +58,11 @@
     'sphinx.ext.intersphinx',
     'sphinx.ext.todo',
     'sphinx.ext.ifconfig',
-    'sphinx_rtd_theme',
-    'sphinx_last_updated_by_git',
 ]
+if has_theme:
+    extensions.append('sphinx_rtd_theme')
+if has_updated_by_git:
+    extensions.append('sphinx_last_updated_by_git')
 
 # See https://github.com/readthedocs/recommonmark#linking-to-headings-in-other-files
 autosectionlabel_prefix_document = True
@@ -95,16 +107,15 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-#
-html_theme = 'sphinx_rtd_theme'
+if has_theme:
+    html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
 #
 html_theme_options = {
     'style_external_links': True
-
 }
 
 # Add any paths that contain custom static files (such as style sheets) here,