Fix #3356 and #3394 latex text layout for Japanese document

[jfbu]

This commit moves the location of geometry package to after
sphinx package is loaded; it adds new keys for 'sphinxsetup'
allowing to easily configure the page margins, by-passing the legacy 1in
for all margins applied by Sphinx. For Japanese documents, the specified
margins are slightly modified in order for text width to be an integer
multiple of the zw dimension unit, and for text height to have an
integer number of baselines. It is always possible to use directly
'geometry' key to latex_elements for complete control, if
desired.

Subject:

Feature and Bugfix

• Feature: allows configuration of text margins from conf.py
• Bugfix: Page layout for Japanese manual docclass has a shorter text area #3356

This PR will slightly alter page layout, the most important change being that the text height for manual docclass gains 2 baselines. (only for Japanese documents...)

Purpose

• straighten out some issues with Japanese text layout and pick this opportunity to make text layout generally speaking more easily customizable.
• the Japanese howto and manual docclass are now handled a bit more similarly.
• the non-Japanese documents gain the ability to specify from conf.py separate margins for left, right, top, bottom, using geometry package syntax (the interface for Japanese documents is more restricted as left and right must be equal and top and bottom too.)

Relates

• Page layout for Japanese manual docclass has a shorter text area #3356
• the parskip package should not be loaded from inside sphinx.sty, but made into a template key, whose loading can be cancelled from conf.py, but this is for another occasion.
• the parskip package does not look really adapted to Japanese typesetting as it introduces vertical shifts of one half baseline. But anyhow Sphinx uses raggedbottom config. But again, this PR does nothing with parskip.
• this PR does also nothing regarding the matter of dvi specials (see Page layout for Japanese manual docclass has a shorter text area #3356 (comment)) final version does after all

[jfbu]

squashed and rebase and now also fixes #3394 issue

[jfbu]

typo here ...Package

[jfbu]

the truedimen option passed to geometry may be located as package option after all, not necessarily as a document class option.

Note: this PR adds the feature of customizability of the document margins. I have also included the width for margin annotations, but perhaps this should be kept undocumented because there is anyhow no reST interface for putting contents in the margins ?

[tk0miya]

Sorry for late. LGTM!
(my time was stolen by nintendo...)

[jfbu]

my time was stolen by colleagues ;-) coincidentally I came back seconds after you completed the review, thanks !

I hesitate a slight bit about the marginpar : should it be documented as as new key to 'sphinxsetup' as I did ? because there is no syntax in reST for margin notes. Admittedly some user may hack LaTeX footnotes to render as margin notes; or some extension of Sphinx could use margin notes for extra annotations. Anyway I will merge it and if you feel it is better to not document the possibility to adjust the width of margin notes, it can always be removed later. Also I will add a note to CHANGES to indicate paper margins are customizable for PDF output.