[Docs] Add version footnote to tor.md

Added note to section 2, part -edits to `/etc/tor/torrc`- indicating this is only required for Tor version 0.2.7.0 and older, since section 3 states it is valid for Tor version 0.2.7.1 and newer. Added ref link from section 2 version footnote to section 3. Re-styled headings to work on GitHub -alternate heading style markup creation issue with numbered headings and thus headings and automatic heading links are broken-

Ref: [Issue# 12376](https://github.com/bitcoin/bitcoin/issues/12376)

Signed-off-by: Damian Williamson \<willtech@live.com.au\>