[Doc][summer-2021] Add TOC and format how-to-contribute-to-apache-carbondata.md
[Doc][summer-2021] Add TOC and format how-to-contribute-to-apache-carbondata.md

Due to Github Flavored Markdown, Github does't support TOC automatic generation in Markdown file. Use  anchors to implement  TOC of headings.

zzcclp/carbondata,zzcclp/carbondata,zzcclp/carbondata,zzcclp/carbondata,zzcclp/carbondata