DOC: Merge FAQ and gotcha
- [x] closes #9809
- [ ]  ToDo:
  - [ ] Add detail of dtype promotion

Moved some topics to corresponding docs. Also removed following duplicates.

**Parsing Dates from Text Files:** the same contents exists on io doc.
- http://pandas-docs.github.io/pandas-docs-travis/gotchas.html#parsing-dates-from-text-files
- http://pandas-docs.github.io/pandas-docs-travis/io.html#specifying-date-columns

**Byte ordering issues:**
- http://pandas-docs.github.io/pandas-docs-travis/faq.html#byte-ordering-issues
- http://pandas-docs.github.io/pandas-docs-travis/gotchas.html#byte-ordering-issues
