docs: update middleware documentation (cosmos#1639) (cosmos#1700)

* docs: update middleware documentation

* remove old text and add func keyword

* alignment

* fix alignment

* Update develop.md

* review comments

* remove empty line

* docs: add links in middleware docs to fee middleware implementation (cosmos#1641)

* docs: add links in middleware docs to fee middleware implementation

* add extra line for better readability

* fix typos

(cherry picked from commit 419c3c4)

Co-authored-by: Carlos Rodriguez <carlos@interchain.io>