Maybe add note about dsd-upsun status to top of "Quick Start: Deploying to Upsun" docs

Context
- First time attempt to use django-simple-deploy
- Minimal prior knowledge about deployment (successfully deployed some toy apps with Heroku a few times following different tutorials so I know "something", but I wouldn't say I left the experiences with any true understanding of the fundamentals)
- I wanted to use django-simple-deploy because I'm still at a level where I'm struggling to read/understand/follow deployment docs straight from the platforms and I saw it supported Upsun which I wanted to use because it's a [B-Corp and looks to be actively trying to reduce environmental impact](https://upsun.com/greener-hosting/)
- I got to the part of the [Quick Start: Deploying to Upsun](https://django-simple-deploy.readthedocs.io/en/latest/quick_starts/quick_start_upsun/) page where I did `upsun create --title ` and then I reread the section [Upsun Flex and Upsun Fixed](https://django-simple-deploy.readthedocs.io/en/latest/quick_starts/quick_start_upsun/#upsun-flex-and-upsun-fixed) and then on another page (I can't seem to find where again but will link here if I can find it - maybe it was in the quote from #17 shown in the last bullet here) read that django-simple-deploy only supports Upsun fixed plans and not flex plans so then I wondered whether or not I would later have the option to switch from fixed to flex in the future so I searched Upsun's docs for the answer to that and instead found this quote on page [Converting from Upsun Fixed (formerly Platform.sh)](https://developer.upsun.com/tutorials/migration/from-fixed), which says:
> Upsun Fixed is no longer available as a new offering and has been removed from the product catalog. This guide is intended for existing Upsun Fixed customers who want to migrate to Upsun Flex. If you are a new customer, start with [Upsun Flex](https://developer.upsun.com/docs/administration/organizations#what-is-an-upsun-flex-organization) directly.
- Seeing that, I figured django-simple-deploy wouldn't work with Upsun right now since its no longer supporting fixed and the dsd-upsun plugin was so far designed specifically for fixed. I wondered if this was known and/or if the process of updating to support flex was underway which is how I came here to the issues and read #17 which said "dsd-upsun only works for Fixed orgs at this point. Don't let people choose Flex orgs, because the deployment will fail." and that @ehmatthes "[Reached] out to Upsun to ask if Fixed plans are being sunset."

Question/Idea
- given #17 saying it only works for fixed and Upsun's docs saying new customers need to start with flex, my understanding is that using django-simple-deploy with dsd-upsun right now isn't an option. Is that correct? (Just confirming to ensure I'm interpreting all this right)
- Assuming the above is correct and since #17 also says "Decide a direction for this plugin based on response." I'm guessing it's still up in the air what will become of dsd-upsun, so I wanted to bring up the idea to add a warning/note/explanation about dsd-upsun's temporary and/or potentially permanent status as "currently not a working option" to the top of the [Quick Start: Deploying to Upsun](https://django-simple-deploy.readthedocs.io/en/latest/quick_starts/quick_start_upsun/) page so that others can know the latest before going down the same info. rabbit hole I went down to realize I couldn't use it

Hope this was helpful; I didn't mean to write a book in the process