Renamed getHeaders() to getHeadings()

[natemoo-re]

What kind of changes does this PR include?

• New or updated content
• Changes to the docs site code

Description

This PR deprecates getHeaders() documentation in favor of getHeadings(). According to the HTML spec, these are headings. The only part of the spec where the word "headers" appear is in the context of

tag. MDN also uses headings to refer the tags.

Depends on withastro/astro#4031

• What does this PR change? Give us a brief description.
• Did you change something visual? A before/after screenshot can be helpful.

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[natemoo-re]

Whoops, looks like there are some merge conflicts still. I'll take a stab at resolving these soon.

[sarah11918]

Suggested change

	The `getHeaders()` function has been renamed to `getHeadings()`. Please update your code to use `getHeadings()`.
	The `getHeaders()` function has been renamed. To update an older project, please use `getHeadings()` instead.

Just a suggestion to be more explicit about which one to use. Similarly, a note about this should end up in the migration guide. (You could probably use this as-is there.)

[sarah11918]

Adding a note that getHeaders() also appears on https://docs.astro.build/en/guides/integrations-guide/mdx/#exported-properties which will need to be updated here: https://github.com/withastro/astro/blob/main/packages/integrations/mdx/README.md

[sarah11918]

@natemoo-re I was easily able to resolve the conflict here, so I did, FYI.

Looks like now it's PageContent and RightSidebar that have some old merge conflict syntax in them?

[delucis]

A lot of these changes to site code have already been handled, so I’m going to close this and raise a new PR with just the docs content changes.