This PR makes the following alterations (mostly suggested by @ctrueden) to the Writing Your Own Op Package page of the documentation:

• Moving the "Best Practices" section of the document to the top, because it really applies to all Ops
• Reorganizing the discovery mechanisms by language, in preparation for future mechanisms operating on other languages 🪄
• Writing a new section for explicit YAML curation (I know, it's not something we'd want to force on people, but I also don't think the YAML format is actually documented anywhere. Maybe it deserves its own page?)
• Adding a disclaimer to the JPMS section that it will be phased out (see Convert all Op plugins from ServiceLoader to op.yaml style #140)

Original list of updates wanted by @ctrueden (I didn't really do either of these, feel free to alter this PR as you see fit!):

• prefer Javadoc over JPMS declaration style
• prefer @OpField for succinctness, or @OpMethod over @opclass if Op dependencies are needed, to minimize boilerplate