boards: heltec_wifi_lora32_v2: improve documentation

* Document use of OLED display.
* Fix indentation in list of features.
* References are only rendered if they are used and currently most of
  them are not. Introduce a list of links in addition to the actual
  references to make them show up.

Signed-off-by: Michael Laß <bevan@bi-co.net>