[DOC](storage policy) add cold and hot separation docs

[deardeng]

Proposed changes

Issue Number: close #xxx

Problem summary

Describe your changes.

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

add cold and hot separation docs

[morningman]

We also need a document to introduce this feature, and best practice of this feature for user. You can refer to

https://doris.apache.org/zh-CN/docs/dev/advanced/cache/partition-cache

[deardeng]

We also need a document to introduce this feature, and best practice of this feature for user. You can refer to

https://doris.apache.org/zh-CN/docs/dev/advanced/cache/partition-cache

I added it.

[morningman]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.