DOC: Link to custom CSS example broken

[pllim]

On page https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/web-components.html , "custom CSS file" that points to https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/custom-css gives a 404

"CSS custom properties" that points to https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/css-variables is also broken.

p.s. Maybe running Sphinx linkcheck could catch some of these ❓

(Noticed this when investigating astropy/astropy#15112 )

[drammock]

"custom CSS file" probably meant to go here: https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/static_assets.html

"css custom properties" probably meant to go here: https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/styling.html#css-theme-variables

also would gladly review/merge addition of linkcheck to our CIs.

[pllim]

#2025 would fix the immediate breakage.

As for linkcheck, maybe I misspoke. I realized now it is intersphinx, not direct URL, so maybe what you need is something like #2026