[DOC] Ensure PPL docs have consistent look

[LantaoJin]

Description

Main updates: Renamed functions/ppl-lambda.md -> functions/ppl-collection.md Move lambda functions to functions/ppl-json.md Refactor ppl-join-command.md, ppl-lookup-command.md, ppl-subquery-command.md

Related Issues

Resolves https://github.com/opensearch-project/opensearch-spark/issues/925

Check List

• [x] Updated documentation (docs/ppl-lang/README.md)
• [ ] Implemented unit tests
• [ ] Implemented tests for combination with other commands
• [ ] New added source code should include a copyright header
• [x] Commits are signed per the DCO using --signoff

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[LantaoJin]

Left comments around using capitalization and back ticks.

Done. Thanks @brijos

[YANG-DB]

@LantaoJin can u plz use this pr to also fix the hyper-link issue here ? thanks !