Cannot buil docs

[pkgzm]

Cannot build the documentation with Debian 12:

.../valum-framework/valum/build$ ninja [55/55] Generating docs/en user documentation with a custom command FAILED: docs/en /usr/bin/sphinx-build -W -b html /home/patrick/src/examples/valum-framework/valum/docs docs/en Running Sphinx v5.3.0

Warning, treated as error: extlinks: Sphinx-6.0 will require a caption string to contain exactly one '%s' and all other '%' need to be escaped as '%%'. ninja: build stopped: subcommand failed.

[arteymix]

We shouldn't build the docs if the rtd theme is unavailable or use the default Sphinx theme as a fallback.