docs: simplify conf.py and fix compatibility with upcoming Sphinx 6.0

Change-Id: Iacf4a72d30babc4a7c3fab0fea619a798eff2df1
diff --git a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
index 2d8d3a5..ade9f8b 100644
--- a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
+++ b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.cpp
@@ -32,8 +32,8 @@
 
 namespace nfd::rib {
 
-const name::Component IGNORE_COMPONENT("nrd");
-const time::seconds DEFAULT_REFRESH_INTERVAL = 25_s;
+const name::Component IGNORE_COMPONENT{"nrd"};
+constexpr time::seconds DEFAULT_REFRESH_INTERVAL = 25_s;
 
 HostToGatewayReadvertisePolicy::HostToGatewayReadvertisePolicy(const ndn::KeyChain& keyChain,
                                                                const ConfigSection& section)