Break up computed observable documentation into multiple pages.
Rename "writeable" to "writable". Expand writable computed docs with additional example and make all examples "live". Various editing to computed docs.
Update writable computed examples to use textInput; add pure computed page placeholder and add pure computed to computed reference.
Update examples to use ko.pureComputed if appropriate.
complete pure computed documentation
typo fix
Merge remote-tracking branch 'origin/gh-pages-3.2.0' into gh-pages-3.2.0-purecomputed

Conflicts:
	documentation/computedObservables.md
typo fix
update isWriteableObservable to isWritableObservable, with note of alias
Merge branch 'gh-pages-3.2.0' into gh-pages-3.2.0-purecomputed
Move 'pure computed' intro down a bit, and emphasise the use case

Reason: For people totally new to KO, it may seem strange and
off-putting to get the discussion about regular-vs-pure computeds right
at the top of the first doc page about computeds. I think it fits
better a bit further down, when a new reader has a better sense of what
a computed is for.
Clarify reason for not using pureComputed for things with side-effects

I know it may seem obvious, but still no reason not to spell it out :)