Include admonitions in inventory

[goerz]

With the addition of anchors for admonitions, it would make sense to also include them in the objects.inv inventory.

These would be in the std domain, label role (same as headings; there isn't really much room in the Sphinx domains to have more semantics). The name would be the anchor name (including the hash), and the dispname would be the title ("Note", "Tip", "Warning", etc. if no title is given).

Presumably, this would require keeping track of these anchors in a field doc.internal.admonitions.map or a similar data structure, akin to doc.internal.headers.map used in the inventory writing.

Not a critical feature, but something nice to have, if I (or someone else) gets around to it.

[goerz]

In theory (if we want the usual consistency between @ref and @extref), that means the admonitions could also be internally linkable via @ref, presumably via the syntax [link text](@ref anchor). The author would have to figure out the auto-generated anchor from hovering over the admonition while viewing the rendered documentation in a browser.