comments inside a grid generate confusing warnings

[feorlen]

Describe the bug

context When I add a comment, either HTML or % style, inside a grid element that uses cards, I get a confusing build warning from Sphinx.

expectation I expected a clean build without warnings. Removing the comment results in no warnings.

bug Here's an error message I ran into...

WARNING: All children of a 'grid-row' should be 'grid-item' [design.grid]

Reproduce the bug

Sample markdown file:

# test grid comments

## this grid works, no comment

::::{grid} 1 2 3 4
:gutter: 3

:::{grid-item-card} AWS app
:::
::::


## this grid gives warning, html comment

::::{grid} 1 2 3 4
:gutter: 3

<!-- foo -->

:::{grid-item-card} AWS app
:::
::::

## this grid gives warning, % comment

::::{grid} 1 2 3 4
:gutter: 3

% foo

:::{grid-item-card} AWS app
:::
::::

Warnings from RTD build log

reading sources... [ 99%] content/workspaces/SearchBookmarksNotifications
reading sources... [ 99%] content/workspaces/apps
reading sources... [100%] index

/home/docs/checkouts/readthedocs.org/user_builds/observe-observe-docs/checkouts/test-grid-bug/docs/content/integrations/index.md:18: WARNING: All children of a 'grid-row' should be 'grid-item' [design.grid]
/home/docs/checkouts/readthedocs.org/user_builds/observe-observe-docs/checkouts/test-grid-bug/docs/content/integrations/index.md:26: WARNING: All children of a 'grid-row' should be 'grid-item' [design.grid]
looking for now-outdated files... none found
pickling environment... done

List your environment

Built with ReadTheDocs.com, Sphinx requirements.txt:

sphinx==5.1.1
six>=1.10.0
myst-parser==0.18.0
furo==2022.06.21
sphinx-copybutton>=0.3.1
sphinx-inline-tabs==2020.10.19b4
sphinx-design==v0.2.0
Pygments>=2.10.0

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[jpmckinney]

I think % comments should work (they should just be ignored when rendering), but HTML comments don't make as much sense. Supporting that is similar effort to supporting arbitrary HTML between grid-item's, which is deliberately unsupported.