Installation: Add libmamba solver to conda Note (napari#110)

# Description

Add mention of using regular conda but with the `libmamba` solver, as a
way to resolve installation issues. See
https://conda.github.io/conda-libmamba-solver/libmamba-vs-classic/
Currently, we suggest mamba. This works well, but breaks copy-paste of
`conda install ...` and other instructions. Changing the solver gives
much of the benefit, without needing to change the command.
See discussion at: napari/napari#5509
 

## Type of change
- [x] Fixes or improves existing content

# References
closes napari#96

## Final checklist:
- [x] My PR is the minimum possible work for the desired functionality

Co-authored-by: Grzegorz Bokota <bokota+github@gmail.com>

docs/_templates/layout.html

@@ -1,5 +1,11 @@
 {% extends "!layout.html" %}
 {% block extrahead %}
     <script async defer data-domain="napari.org" src="https://plausible.io/js/plausible.js"></script>
-    {{ super() }}
 {% endblock %}
+
+{% block document %}
+    <p>PAGENAME: <p>
+{% endblock %}
+
+
+{{ super() }}

docs/conf.py

@@ -174,6 +174,16 @@
    "conda_create_env": f"```sh\nconda create -y -n napari-env -c conda-forge python={python_version}\nconda activate napari-env\n```",
 }
 
+# def linkify_prs(input):
+#     prs = re.compile("#(?P<id>[0-9]+)")
+#     return re.sub(prs, "https://github.com/napari/napari/issues/%(id)s", input)
+
+# linkifyers = {
+#     "(?P<id>[0-9a-f]{7,40})": "https://github.com/napari/napari/commit/%(id)s",
+#     "(?P<org>[a-zA-Z0-9_-]+)/(?P<repo>[a-zA-Z0-9_-]+)#(?P<id>[0-9]+)": "https://github.com/%(org)s/%(repo)s/issues/%(id)s",
+# }
+# Is it possible to define pure substitutions with #N instead of {{ #N }}?
+
 nb_output_stderr = 'show'
 
 panels_add_bootstrap_css = False
@@ -236,6 +246,7 @@ def setup(app):
     """
     app.registry.source_suffix.pop(".ipynb", None)
     app.connect('linkcheck-process-uri', rewrite_github_anchor)
+    #app.connect('release-notes-links', release_notes_links)
 
 
 def get_attributes(item, obj, modulename):