R1:68762e820417

Drydock and Almanac: add welcome pages

Summary:
Premising that I still consider Almanac, Drydock and Harbormaster as dark magic, and premising that all my knowledge about these components is here:

https://en.wikibooks.org/wiki/Phabricator_Administrator%27s_Handbook/Continuous_integration

(Above guide linked from [[Community Resources]])

Hoping to be useful, I've added some welcome pages, precious to newcomers, with the goal to lower a bit the learning barrier to these tricky components.

## Drydock…