[Adalennis]

Fixes #8979

Rewrites templates/template-plugin.adoc (the starting point for plugin pages, according to the developer manual's "Writing documentation" page) so it can be copied as is:

• A short comment on how to use it: where to copy it, what to replace, adding the page to the navigation, and how the plugin's documentationUrl relates to the page.
• The header attributes the existing pages use, each with a comment: :documentationPath:, :language:, :description: (required by the docs PR build since Fail the docs PR build when a page has no description. #2606 #8957), :page-engines: for transforms only, with the meaning of yes / no / maybe / native, and :page-marketplace: for marketplace plugins only.
• The title with the icon for a transform, and a note that an action page has no icon.
• Description and Options sections with the options table and its first row, for transforms and actions. The existing writing tips are kept, in the Options section where they apply.
• The optional sections used on several pages.

All of it follows what the 276 transform pages and 84 action pages have in common. One template is kept rather than two, so the reference from the developer manual still holds.

• Run mvn clean install apache-rat:check to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If you have a group of commits related to the same change, please squash your commits into one and force push your branch using git rebase -i.
• Mention the appropriate issue in your description (for example: addresses #123), if applicable.

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• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004
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[bamaer]

Thanks @Adalennis, this makes the template usable as is. I checked the attributes, title, first rows and comments against the existing transform and action pages, the description check in the docs PR build and the hop-website README; it all matches. Not rendered, since the template isn't published.

One small suggestion: for a transform, the copy instructions could also mention adding the page to the list in pages/pipeline/transforms.adoc, not just nav.adoc. That list has drifted (248 of 279 transform pages; e.g. watchfiles and extractgraph are missing).

LGTM otherwise.