Plugin introduction doc improvements

[stacey-gammon]

Taking some advice from https://documentation.divio.com, I think we should pull the code snippets out of https://elasticdocstest.netlify.app/kibana-dev-docs/platform-intro and into a new tutorials section.

In addition, I added information on extensions and registries to the platform/plugin intro.

Before

This introduction file included quite a bit of code samples and details.

After

Put the code samples into a plugin tutorial

Clean up and add some extra content on the intro

Added content on extensions and registries as these are basic pluggability concepts and words we use a lot.

[alexh97]

I really like this direction. In general, when I'm trying to learn a new system/codebase/architecture, I want to see a very high-level description (preferably with diagrams) first. What are the major domains and building blocks? How do they relate to each other? Why should I care?
Only once I understand the big picture, do I want to dive into code examples. And then I want to use the reference once I'm actively developing.

I think this new structure works much better.

[stacey-gammon]

@elasticmachine merge upstream

[rudolf]

I like the guide vs tutorial split 👍 but left some thoughts on the platform vs core distinction

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 17f3cda
• Documentation Changes

Metrics [docs]

✅ unchanged

History

• 💔 Build #105171 failed 031af9e
• 💔 Build #105183 failed 773964f
• 💚 Build #104866 succeeded 247bdf5
• 💚 Build #104398 succeeded 0cc8af9

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[kibanamachine]

Backport result

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{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git remote rm elastic","stdout":"","stderr":"error: No such remote: 'elastic'\n"},"level":"info","message":"exec error 'git remote rm elastic':"}
{"level":"info","message":"Backporting [{\"sourceBranch\":\"master\",\"targetBranchesFromLabels\":[\"7.x\"],\"sha\":\"10adb72ddbf56b7d3a060315b9559900bea2adfc\",\"formattedMessage\":\"Plugin introduction doc improvements (#90502)\",\"originalMessage\":\"Plugin introduction doc improvements (#90502)\\n\\n* More plugin docs improvements\\r\\n\\r\\n* remove speculation about the future\\r\\n\\r\\n* remove registries section\\r\\n\\r\\nCo-authored-by: Kibana Machine <[email protected]>\",\"pullNumber\":90502,\"existingTargetPullRequests\":[]}] to 7.x"}

Backporting to 7.x:
{"level":"info","message":"Backporting via filesystem"}
{"level":"info","message":"Creating PR with title: \"[7.x] Plugin introduction doc improvements (#90502)\". kibanamachine:backport/7.x/pr-90502 -> 7.x"}
{"level":"info","message":"POST /repos/elastic/kibana/pulls - 201 in 1138ms"}
{"level":"info","message":"Adding assignees to #90854: stacey-gammon"}
{"level":"info","message":"POST /repos/elastic/kibana/issues/90854/assignees - 201 in 656ms"}
{"level":"info","message":"Adding labels: backport"}
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View pull request: https://github.com/elastic/kibana/pull/90854