doc build: add optional sphinx_rtd_theme

The theme is no longer included in newer Sphinx versions,
so some systems (at least) will require specifying it explicitly.
(The docs refuse to build without it.)

diff --git a/doc/build.rst b/doc/build.rst
index 6b37767ae137381062cf9c20de329077801a71e2..270f41debd04b867197e253ef55dd39f9e5962f4 100644 (file)
--- a/doc/build.rst
+++ b/doc/build.rst
@@ -52,7 +52,7 @@ There are also *optional* packages that enable specific functionality in Knot DN
    "Go_ 1.5+", "``modules``", "Build modules written in Go."
    "cmocka_", "``unit tests``", "Unit testing framework."
    "Doxygen_", "``documentation``", "Generating API documentation."
-   "Sphinx_", "``documentation``", "Building this HTML/PDF documentation."
+   "Sphinx_ and sphinx_rtd_theme_", "``documentation``", "Building this HTML/PDF documentation."
    "breathe_", "``documentation``", "Exposing Doxygen API doc to Sphinx."
    "libsystemd_", "``daemon``", "Systemd socket activation support."
 
@@ -276,6 +276,7 @@ You can hack on the container by changing the container entrypoint to shell like
 .. _Doxygen: https://www.stack.nl/~dimitri/doxygen/manual/index.html
 .. _breathe: https://github.com/michaeljones/breathe
 .. _Sphinx: http://sphinx-doc.org/
+.. _sphinx_rtd_theme: https://pypi.python.org/pypi/sphinx_rtd_theme
 .. _GNU Make: https://www.gnu.org/software/make/
 .. _pkg-config: https://www.freedesktop.org/wiki/Software/pkg-config/
 .. _libknot: https://gitlab.labs.nic.cz/labs/knot