[doc] Center branding text in sidebar

Most recent documentation builds no longer center the text in the
sidebar. Instead, it's aligned to the left. This is probably due
to changes in Sphinx 6.0.0 that broke a variable Furo relies on
to center that text. I was not able to pinpoint the root cause so
I'm proposing custom CSS as a workaround. Tested with Sphinx 7.2.3
and Furo 2023.09.10.

Change-Id: Ib33c97355cfe2b696d72c5520bcd69a2c691bfec
---
M docs/_static/css/pywikibot.css
1 file changed, 21 insertions(+), 0 deletions(-)

diff --git a/docs/_static/css/pywikibot.css b/docs/_static/css/pywikibot.css
index b03fa36..80e0450 100644
--- a/docs/_static/css/pywikibot.css
+++ b/docs/_static/css/pywikibot.css
@@ -5,6 +5,11 @@
     max-width: 75%;
 }
 
+/** centering sidebar branding text as a workaround **/
+.sidebar-brand {
+    text-align: center;
+}
+
 /** brand text in the sidebar smaller than default **/
 .sidebar-brand-text {
     font-size: 1rem;

