1. Upgraded codebase to use more plDoc v2 Markdown syntax. 2. Documented DCG_YEAR.

* [SC] Use Markdown headers everywhere.
* [DOC,SC] Use Markdown code segment indicators with file types / extensions everywhere.
* [DCG_YEAR,DOC] Added documentation on preposition intervals.
* [DCG_YEAR,SC] Removed year_point//2 and year_interval//2 from the public predicates list.
* [ILP,FIX] Small fixes.