Add mention of composite actions under "Types of actions"

[mhucka]

The first sentence of the paragraph under the heading "Types of Actions" only mentioned Docker and JavaScript actions, but the table right below that paragraph contains 3 types of actions. The first sentence of the paragraph could be modified to mention composite actions to make it more complete.

Why:

Closes #22096.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Change the first sentence of the paragraph under the heading "Types of Actions" to mention composite actions.

Check off the following:

• [x] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [x] For content changes, I have completed the self-review checklist.

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Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/creating-actions/about-custom-actions.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae

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fpt: Free, Pro, Team ghec: GitHub Enterprise Cloud ghes: GitHub Enterprise Server ghae: GitHub AE

[cmwilson21]

@mhucka Thanks for submitting a PR! I made a slight change to your post to link it to your issue. 👍

I'll get this triaged for review ⚡

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