[v3] doc creation/installation rules

[Benjamin Kosnik]

Adds the following rules as per the GNU Coding Standards:
html, pdf, man, info, ps, dvi

Adds as extensions:
xml, epub

And associated install rules.

Using the same approach used for makeinfo, these rules are
conditionalized based on the existence of the necessary dependencies.
These conditions are pretty strict at the moment.

Note that the existing private rules, the ones prefixed with doc-, are
always available. 

tested x86_64/linux

-benjamin
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