Add an easy template for index (i)

Add an easy template for index (i)

* lisp/org.el (org-structure-template-alist):  Add an easy template
for index (i), and move include file to I from i.
* doc/org.texi (Easy Templates): Document new template.

Notes about this patch:
1.  It breaks some old user-visible behavior, since <i changes meaning.
    Per Nick's posting, we expect that if index is used, it will be used
    more commonly than include file.  However, since this is a custom,
    behavior could be changed.  Indeed, we could put index on capital
    I with a suggestion that users who are indexing should swap in their
    customizations.
2.  I modified the docstring for org-structure-template-alist, which did
    not explain the function of the "?" in the string.  Someone should
    check and verify I didn't get this wrong.
3.  There doesn't seem to be a Muse tag equivalent for #+index, so I
    just made the Muse equivalent of #+index be #+index.  I don't know
    org-mtags enough to know if this is appropriate.