docs: add a connector page for on-premises SharePoint Server

[marevol]

The SharePoint connector had no page in this documentation. In seven
languages its only mention was one row of the Japanese overview table,
describing it as a legacy version.

That description is wrong, and it is wrong in a way that misleads. The
Microsoft 365 connector uses the Graph API and reaches SharePoint Online
only; for an on-premises SharePoint Server farm this connector is not an
older alternative, it is the only one. A reader who found the overview row
and moved on would conclude, incorrectly, that Fess cannot crawl their farm.

Adds ds-sharepoint.rst to the 15.9 tree in all seven languages, covering
what previously had to be read out of the source: the full parameter set
including authentication, the script variables each crawl type produces,
how permissions are resolved, the managed-path and subsite options, the
parallel-crawl and throttling behaviour, four complete configuration
examples, and the limitations - among them that there is no incremental
crawl, that IIS Extended Protection set to Require cannot be supported,
and that data-config passwords are stored and displayed in clear text.

The overview table and the datastore toctree gain the page in all seven
languages, so the six that never mentioned this connector now do. The
Japanese overview's legacy row is replaced by the linked entry; its
Wikipedia row is left alone. The Microsoft 365 page gains a pointer for
readers who land there looking for an on-premises farm, and the new page
points back for readers who want SharePoint Online.

Only the 15.9 development tree is touched.