* CLICKHOUSEDOCS-733: Fixed some broken links. Updated introduction and TOC in aggregate functions.

* CLICKHOUSEDOCS-733: Fixed files endings.

* CLICKHOUSEDOCS-733: Returned the list of functions back to reference index.
Co-authored-by: NSergei Shtykov <bayonet@yandex-team.ru>
Co-authored-by: Nemironyuk <em@don.ru>