docs: rename 'compactor' to 'summarizer' in documentation

[csy1204]

Updated documentation to replace 'compactor' with 'summarizer' and adjusted references accordingly.

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[koverholt]

Thanks for the fix and PR! ❤️

Confirming that the updates are correct. I made a few formatting fixes elsewhere in the file, and everything LGTM.

[koverholt]

@csy1204: You'll need to sign a Contributor License Agreement (CLA) before this can be merged. Please refer to the CI test output here for more info: https://github.com/google/adk-docs/pull/1007/checks?check_run_id=56844324325.

[csy1204]

@koverholt Thank you for noticing!