Add more prominent link to rendered docs

[ml-evs]

We have this section in our documentation (https://www.optimade.org/optimade-python-tools/configuration/#environment-variables) but perhaps it would be nice to link to it in the script itself, I agree the distinction between uvicorn flags/OPTIMADE options is a bit confusing.

Ah, thanks, I had indeed missed this. In fact, I had missed that there was a pre-rendered version of the full docs at https://www.optimade.org/optimade-python-tools/, (but I see I could have picked up the configuration info from reading `docs/configuration.md".)

Perhaps in the overhaul of the docs once could consider a slightly more prominent link to https://www.optimade.org/optimade-python-tools/ in the README.md to help direct those who glances over it, e.g. as:

# Documentation 

Full documentation is available at the [optimade-python-tools website.](https://www.optimade.org/optimade-python-tools/)

(But I see how this link gets a bit weird if you want the first page of the website to be rendered from the README.md, hmm...)

Originally posted by @rartino in #627 (comment)