docs(jaeger): update the doc how to use jaeger

[FANNG1]

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Summary

after #6340, "DATABEND_JAEGER" is replaced by "DATABEND_JAEGER_AGENT_ENDPOINT" , we should update the doc and show how to use it. for a newbie, may do't know how to set "DATABEND_JAEGER_AGENT_ENDPOINT"

[vercel]

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Name	Status	Preview	Updated
databend	✅ Ready (Inspect)	Visit Preview	Jul 5, 2022 at 0:46AM (UTC)

[mergify]

This pull request's title is not fulfill the requirements. @sandflee please update it 🙏.

Valid format:

fix(query): fix group by string bug
  ^         ^---------------------^
  |         |
  |         +-> Summary in present tense.
  |
  +-------> Type: feat, fix, refactor, ci, build, docs, website, chore

Valid types:

• feat: this PR introduces a new feature to the codebase
• fix: this PR patches a bug in codebase
• refactor: this PR changes the code base without new features or bugfix
• ci|build: this PR changes build/testing/ci steps
• docs|website: this PR changes the documents or websites
• chore: this PR only has small changes that no need to record

[BohuTANG]

👍