Document how to create ASGI middlewares (#1656)

Document how to create ASGI middlewares (#1656)

* Document how to create ASGI middlewares

* Fix typo

* Add more documentation on how to create pure ASGI middlewares

* Apply suggestions from code review

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>
* Apply suggestions from code review

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
* remove empty spaces

* Add asgiref hyperlink

* Fix per-request section

* Fix scope info

* Remove example section

* Update docs/middleware.md

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>
* Update docs/middleware.md

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>
* Apply suggestions from code review

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>
* Update docs/middleware.md

Co-authored-by: Jean Hominal <jhominal@gmail.com>
* Apply suggestions from code review

* fix typo

* Apply suggestions from code review

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
* Apply suggestions from code review

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
* Apply suggestions from code review

* Update docs/middleware.md

* Add reference to Starlette components

* Add `MutableHeaders` import

* Fix typos from suggestions

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
Co-authored-by: Adrian Garcia Badaracco <1755071+adriangb@users.noreply.github.com>
Co-authored-by: Jean Hominal <jhominal@gmail.com>