Multiple improvements to Alertmanager configuration docs

* The current page outline was an unstructured and unsorted mess, so I tried to
  organize the different configuration file fields into categories.
* I also sorted receivers alphabetically.
* Corrected the Telegram receiver's "bot_token" to be a "secret", not "string".
* Other minor improvements and wording additions to the sections.

Signed-off-by: Julius Volz <julius.volz@gmail.com>