Clean up extension template and docs - part 2

[hacksparrow]

As a follow up task of #5336, the following needs to be done.

• Use the component name in the file names to disambiguate them from similar files in other extensions, e.g. src/{name}.keys.ts and src/{name}.component.ts
• Review the content in the pages above and in the component migration guide (https://loopback.io/doc/en/lb4/migration-extensions.html and sub-pages). Some content may be duplicated (sometimes inconsistently), some content may belong to a different place. Decide what to do (if anything) and either make the changes right away or create follow-up issues.
• Document the extension project layout as docs: add project layout ref page #6151 does for applications.

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