Add codeowners for doc image files and rule maintainers

What does this MR do and why?

While reviewing !97298 (merged), I noticed that it needed a code maintainer approval, despite being docs-only. I realized that .png images are not taken into account in the codeowners, so any MR updating a docs image falls back on the default * approval rule (needing a mandatory code maintainer approval). TWs are not included as code maintainers, so they cannot approve or merge docs MRs with images right now.

This MR adds *.png to the rules for docs files. Adding /doc/*.png is not needed because we do not want to put any images in the root of the docs directory tree.

Additionally, the TW team's codeowners entries change from time to time when we reassign TWs to different groups, so we should add Technical Writing Leadership team members to the list of users who can approve changes to the file. Discussed here: !97072 (comment 1090253124)

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