Clean `customize.md` file

[giladturok]

• Consider adding table of contents
• I often specify which file (cleancv.sty, publications.bib, or main.tex) to update to implement a specific feature. Be consistent in (1) including this info and (2) mentioning it in the same place e.g. in plaintext or in the code example
• Update heading sizes
• Better "call to action" to use the customize.md guide from the README
• Explain how to add custom contact bar symbols from FAIcons / AwesomeIcons