docs: fix bad advice in the README

Do not recommend running the 'upload' target by default in order to
build the docs. That will fail with permission errors when trying to
push to a repo most people don't have commit access to, and if they did
have commit access it would be an even worse problem -- unpredictably
overwriting the main website without any guarantee it was generated from
the latest version of the docs!

Plus, it does not actually work. The first thing it does is spawn an
error message that required files do not exist, because the actual docs
were not, in fact, built. So they cannot be uploaded either.

The docs will build by default if you do not specify a non-default
target.

Fixes #9873

docs/README.md

@@ -13,8 +13,8 @@ Instructions on how to install hotdoc are [here](https://hotdoc.github.io/instal
 From the Meson repository root dir:
 ```
 $ cd docs/
-$ meson built_docs
-$ ninja -C built_docs/ upload
+$ meson setup built_docs/
+$ ninja -C built_docs/
 ```
 Now you should be able to open the documentation locally
 ```