Create TOC for class methods, cleanup titles (#7630)
Create TOC for class methods, cleanup titles (#7630)

* Create TOC for class methods, cleanup titles

Finding content within the doc is a pain, so adding a link to their sections in the markdown should help. Also styled the class methods in the headings on the page.

* Update models-usage.md

sequelize/sequelize,Americas/sequelize,overlookmotel/sequelize,sequelize/sequelize,grimurd/sequelize,inDream/sequelize,jayprakash1/sequelize,Alexsey/sequelize,gorangajic/sequelize,Americas/sequelize,yjwong/sequelize,jayprakash1/sequelize,PhinCo/sequelize,majorleaguesoccer/sequelize,cbauerme/sequelize,inDream/sequelize,yonjah/sequelize,Wsiegenthaler/sequelize,pola88/sequelize,konnecteam/sequelize,sequelize/sequelize,overlookmotel/sequelize,p0vidl0/sequelize,nemisj/sequelize,PsiXdev/sequelize,konnecteam/sequelize,mikeringrose/sequelize,Verdier/sequelize