Update style for H4 headings

[snneji]

Issue

H4 headings look like bold text. It would be nice for these headings to have a distinctive style.

Example

Screenshot of H4 formatting:
(https://knative.dev/docs/developer/eventing/sources/sinkbinding/reference/#cloudevent-overrides)

Resources

The CSS file for customizing the layout can be found here: https://github.com/knative/docs/blob/mkdocs/docs/stylesheets/extra.css

[snneji]

/kind mkdocs
/kind good-first-issue

[doitpratik]

I would like to work on this issue, @snneji can you please assign this issue ?

[snneji]

/assign @soham283

[snneji]

Hi @soham283, I assigned you to this issue. I believe you can also assign yourself to issues using the /assign command in a comment. The Knative prow robot then assigns you to the issue.

[doitpratik]

please review the pr #3767 @snneji and @omerbensaadon

[snneji]

Done in #3767