add Developer Platform instructions (#32481)

* add Developer Platform instructions

* add testing section

* add tabs for ootb vs. marketplace

* rewrite more sections

* fix formatting

* fix link references

* minor formatting fix

* fix vale warnings

* Update content/en/developers/integrations/agent_integration.md

Co-authored-by: Erica Ho <[email protected]>

* Update content/en/developers/integrations/agent_integration.md

Co-authored-by: Erica Ho <[email protected]>

* address feedback

* clarify sample_tags format

* address feedback

* add traces note

* fix capitalization

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Co-authored-by: Erica Ho <[email protected]>

Author: dd-dominic