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`

Look at the 'Create Index' and 'Create Table' documentation to understand current TTL coverage. Examine the code for index creation and deployment logic to see how TTL is set and updated. Review any existing restrictions on TTL types and window definitions. The goal is to produce a new document or blog post explaining TTL behavior and auto-indexing.

索引モデルが issue の本文から書いたものです。