feat: SDXS-09 support and update doc

[akleine]

1. Download links have been provided instead of the conversion process.
2. The ckpt references have been replaced with those on SafeTensors.

These changes were made because this file is intended for less experienced users. They are not required to use *.ckpt and can download the models instead of creating them themselves.

[akleine]

After some updates to the document file (committed yesterday)
here comes again some code for supporting SDXS-09 .

We have some users who are eagerly awaiting this SDXS-09 patch, e.g. @ProgerXP .
But since February 5th, a lot of code has been changed, leading to merge conflicts.

Therefore, I have updated my old SDXS-09 code to reflect the current state.
For this reason the old PR #1254 will be closed soon.

Also to better distinguish between both SDXS versions, the "old" VERSION_SDXS is now
called VERSION_SDXS_512_DS, where DS stands for the DreamShaper edition by IDKiro.

A general note: Although the name "SDXS-09" is similar to SDXS-512-DS, the model is completely different,
but also similarly ultra-fast.

This makes it interesting for many users. And unlike the SDXS-512-DS, SDXS-09 generates more different images
for the same prompt, depending from used seed value.