docs: update Windows Stemcell OS page
docs: update Windows Stemcell OS page

- Add 1803 and 2019 distributions
- Add reference to `stembuild`
- Document deprecation of `bosh-windows-stemcell-builder`

Reference to the deprecated repository is kept for posterity as:

- There are no replacements for building OpenStack Windows Stemcells and
- There are no other references to `bosh-windows-stemcell-builder`

Signed-off-by: Rifa Achrinza <a0099bd8297a390b8df8a1ec606ffb7d3d367f1f@users.noreply.github.com>

cloudfoundry/docs-bosh