[doc] dev - cleanup asdf remnants and add a note about mise-en-place (#5264)

Notes on the developer environment with MISE have been added to the Quickstart
Guide.  We migrated `.tool-versions` to replace it with `mise` [1], so we can
now remove `asdf` related documentation.

[1] https://github.com/searxng/searxng/pull/5253

Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>

docs/dev/quickstart.rst

@@ -23,6 +23,24 @@ Don't hesitate, just clone SearXNG's sources and start hacking right now ..
 
     git clone https://github.com/searxng/searxng.git searxng
 
+For the developer environment, `mise en place`_ is recommended:
+
+.. code:: sh
+
+   $ curl https://mise.run | sh
+
+The tools required for a developer environment are provided via `mise.toml`_,
+trust the project:
+
+.. code:: sh
+
+   $ mise trust
+
+.. _mise.toml:
+    https://github.com/searxng/searxng/blob/master/mise.toml
+.. _mise en place:
+    https://mise.jdx.dev/getting-started.html
+
 Here is how a minimal workflow looks like:
 
 1. *start* hacking