🐛 fix(docs): fix ReadTheDocs build

The RTD build failed because the trailing `--` in the tox invocation set
posargs to empty, causing sphinx-build to receive no output directory.

Remove the `--`, let tox use the default output path, and copy the
result to `$READTHEDOCS_OUTPUT/html` where RTD expects it. Also fix
two RST title underlines that were too short, which caused failures
under `-W` (warnings as errors).

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>

Author: gaborbernat

.readthedocs.yaml

@@ -5,4 +5,4 @@ build:
   commands:
     - curl -LsSf https://astral.sh/uv/install.sh | sh
     - ~/.local/bin/uv tool install tox --with tox-uv -p 3.14 --managed-python
-    - ~/.local/bin/tox run -e docs --
+    - ~/.local/bin/tox run -e docs -- "$READTHEDOCS_OUTPUT/html"

docs/explanation.rst

@@ -2,7 +2,7 @@ How it works
 ============
 
 Where does python-discovery look?
--------------------------------
+---------------------------------
 
 When you call :func:`~python_discovery.get_interpreter`, the library checks several locations in
 order. It stops as soon as it finds an interpreter that matches your spec.

docs/index.rst

@@ -1,5 +1,5 @@
 python-discovery
-============
+================
 
 You may have multiple Python versions installed on your machine -- system Python, versions from
 `pyenv <https://github.com/pyenv/pyenv>`_, `mise <https://mise.jdx.dev/>`_,