tex-xskak{,-doc}: update to 1.5

- The source of the documentation has been added, so that
  it can now be included in TeX Live.

- A missing command has been added.

- A missing space in a style has been added.

- A few typos in the documentation have been corrected.

- Examples for the predefined styles have been added.

- References to older skak versions have been removed.

- The code is now on github.

(markd)