chore(kno-11556): Update Step component to add h4 support and heading size alignment

[rachael-t]

Description

I noticed that step headings weren't appearing in this section of the docs, and after digging in realized that this was because the titleSize was set to h4 and the Step component was returning null in this case. To have better support moving forward, I've added the following changes:

1. Explicitly added support for titleSize="h4" (previously there was only support for "p", "h2" and "h3"
2. Added a TitleSize union type ("p" | "h2" | "h3" | "h4") and typed all component props accordingly so that passing an invalid titleSize is now a compile-time error rather than a silent runtime failure
3. I kept default: null (which is reverted from the previous version of this PR) as it looks like this was intentional original behavior since Steps defaults titleSize to "p" at the prop level, meaning default was never meant to be reached. With the TitleSize type now enforced, it genuinely can't be reached, and a comment has been added to reflect that
4. Updated the heading sizes to align with the markdown headings. Step titles now use the same Telegraph Heading size as SectionHeading (used for ##, ###, #### in MDX via mdxComponents) to help keep the step headings consistent with the rest of the docs.
5. I also removed backticks from the access_token step title in sending-a-direct-message.mdx as currently backticks don't render as inline code in the title prop and were displaying as literal characters. Worth us considering adding inline code parsing to the component, but this was the only use of backticks in a title in the docs, so I opted to just remove it from this one use case for now.

Tasks

KNO-11556

Screenshots

Before:

After:
Can be seen under this section after the "We'll break this function down step-by-step:"

^{Cursor Bugbot found 1 potential issue for commit 48a6f48}

[linear]

KNO-11556 [Docs] Fix step titleSize on Slack DM page

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview, Comment	Jul 9, 2026 9:22pm

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[scotidodson]

Didn't see this in the PR description so just making sure you intend to include this?

[rachael-t]

Yes, sorry I didn't add this! I just updated the description to clarify that this was intentionally removed.

[cellomatt]

One tiny thing but this lgtm