Docs: Remove redundant doc files, preserve changes

Docs: Remove redundant doc files, preserve changes

When the web client was first released, some sections of the docs were
copied to provide both a XUL and a browser client version of the docs.
The continued existence of these mostly duplicate files has caused much
confusion. This commit attempts to bring final resolution by deleting
the original XUL files, while also copying in any improvements they had
compared to the web client files. Here's a summary of the main changes
in this commit:

  - Deleted these XUL doc files
      - circulating_items.adoc

Changes in circulating_items_web_client.adoc:

  - Lots of line reformatting (breaking long lines at 80 characters)
  - Add keyboard shortcuts (F1, etc.)
  - New section "Items Out Display Settings" based on content in deleted file
  - Replaced "Item Alerts" section with new "Copy Alerts" docs (new in 3.1)
  - Small improvements to "Long Overdue Items" section

STILL TO DO:

  - Compare circulation_patron_records.adoc with the web client version,
    and copy valuable differences into the web client version.
    NOTE: It may help to check the commit history of
    circulation_patron_records.adoc (or its previous .txt filename).
  - Delete circulation_patron_records.adoc

Signed-off-by: Remington Steed <rjs7@calvin.edu>