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

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13768?src=pr) is 85.25% (diff: 100%)

> Merging [#13768](https://codecov.io/gh/pydata/pandas/pull/13768?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13768   diff @@
==========================================
  Files           140        140          
  Lines         50455      50455          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43014      43014          
  Misses         7441       7441          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [59f2557...8fcb152](https://codecov.io/gh/pydata/pandas/compare/59f25575f42e19f066dc3cd2245983071e2d7813...8fcb15260d2da09af2a7a0142b39fce687edf74e?src=pr)
 As @jorisvandenbossche mentioned in the issue discussing consolidating these pages, I would suggest removing the FAQ page and keeping the "Gotchas" page instead. In my opinion, most of these are a better fit there.

The only exception is "DataFrame memory usage" which should probably be moved somewhere else (it isn't a great fit in the FAQ, either). Maybe "Internals" or "Enhancing Performance"?
 @jorisvandenbossche ?
 As I said in the original issue, although I like the FAQ name, I have the feeling that the content is not really what you would expect from a pandas FAQ (I would expect to see general things like 'what is pandas?', 'how can I contribute?', 'how is pandas developed?', 'how does pandas depend on numpy?', ...), while this are more advanced "gotcha's".
 @jorisvandenbossche @sinhrks status on this?
 where are we on this?
 @sinhrks can you rebase status of this? @sinhrks rebased this + made some changes for the comments above -> https://github.com/pandas-dev/pandas/pull/15222