docs about ttl and auto index

**Describe the feature you'd like**

Currently there is no place discussing about how the index ttl will be updated / set in our system. We may need a blog/page discussing how it works. It may include:
- how user define an index with ttl (it is currently covered in the `Create Index` and `Create Table`)?
- how the existing ttl (user defined) may be changed by the system? Upon a deployment, there may be an update on the ttl if the window is larger than existing ttl.
- there is also some restrictions. We may discuss it is reasonable to have such restrictions.
- If ttl is defined as `absolute`, `ROWS` window is not allowed.
- if ttl is defined as `latest`, `ROWS_RANGE` window is not allowed
- how we auto create indexes and set the ttl when `deploy`