Enhancement: Follow the same templates and statics folder layout with Flask Admin

[hasansezertasan]

Is your feature request related to a problem? Please describe.
I (and many others IMO) intend to use a single templates instance for their projects. Starlette Admin occupies some file names (that are very common) and conflicts occur. For example, base.html and index.html are quite standard file names for templates.

Also following the flask-admin's approach would make it easy to have more UI options.

- templates
  - tabler1
    - admin  # Current templates folder goes here
  - tabler2
    - admin
  - bootstrap5
    - admin
  - tailwind  # I would like to work on it in the future!
    - admin
  - ...

Describe the solution you'd like
Follow the Flask Admin layout:

Statics and Templates

[hasansezertasan]

One other thing I would like to add is Flask Appbuilder's approach:

Flask-AppBuilder/flask_appbuilder/templates/appbuilder at master · dpgaspar/Flask-AppBuilder

So for this layout, we would have to update the file structure like this:

- templates
  - starlette_admin
    - tabler1
      - admin  # Current templates folder goes here
    - tabler2
      - admin
    - bootstrap5
      - admin
    - tailwind
      - admin
    - ...

[hasansezertasan]

Related: aminalaee/sqladmin#747
Solution: aminalaee/sqladmin#748