chg: doc: Disable strict "turn warnings into errors" Sphinx mode
Otherwise documentation build fails with:
/builds/isc-projects/bind9/doc/arm/general.rst:413: WARNING: Footnote [#] is not referenced. [ref.footnote]
/builds/isc-projects/bind9/doc/arm/general.rst:415: WARNING: Footnote [#] is not referenced. [ref.footnote]
/builds/isc-projects/bind9/doc/arm/general.rst:419: WARNING: Footnote [#] is not referenced. [ref.footnote]
/builds/isc-projects/bind9/doc/arm/general.rst:423: WARNING: Footnote [#] is not referenced. [ref.footnote]