[DOCS][REFACTOR] Organize Design and Architectures (#6097)

[DOCS][REFACTOR] Organize Design and Architectures (#6097)

* [DOCS][REFACTOR] Design and Architectures

This PR refactors the design and architecture docs.
Previously this part of documentation was quite unstructured, and lacks a global
view of the overall architecture.

This PR takes a stab in resolving the problem

- Provide a guided overview of the current TVM's overall design
- Categorize the specific docs into architecture components or How tos.

* Apply suggestions from code review

Co-authored-by: Jared Roesch <roeschinc@gmail.com>
* Apply suggestions from code review

Co-authored-by: Jared Roesch <roeschinc@gmail.com>
* Update per comment

* More updates per feedbacks

* clarify external codegen

* Update per comments

Co-authored-by: Jared Roesch <roeschinc@gmail.com>