doc: Migrate to Linaro release 19.06

- Updated Linaro release version number to 19.06
- Updated links to Linaro instructions and releases
- Removed the Linaro old releases link

Signed-off-by: zelalem-aweke <zelalem.aweke@arm.com>
Change-Id: Ib786728106961e89182b42183e7b889f6fc74190