Adjust docs for Sphinx v6

[khaeru]

Sphinx 6.0.0 was released on 2022-12-29; since then, the CI step "Test documentation build using Sphinx" has failed, e.g. here, with:

writing output... [  5%] api

Exception occurred:
  File "/opt/hostedtoolcache/Python/3.11.1/x64/lib/python3.11/site-packages/pydata_sphinx_theme/bootstrap_html_translator.py", line 27, in visit_table
    self.generate_targets_for_table(node)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'BootstrapHTML5Translator' object has no attribute 'generate_targets_for_table'

Investigate and adjust.

[khaeru]

On closer inspection this appears to be pydata/pydata-sphinx-theme#1098. Until this is closed, the mitigation may be to require sphinx < 6.0.0.

[khaeru]

pydata-sphinx-theme is required via sphinx-book-theme. The latter package has been adjusted to exclude Sphinx 6 (executablebooks/sphinx-book-theme#644) but this has not yet been released. Until that happens, sdmx1 will need to mirror that exclusion → #113