docs: when using venv with a clone, recommend using 'venv' as location

Avoid having the odd inconsistent location ../kallithea-venv to care about.

For good or bad, this might change how it is feasible to grep. I usually make
sure to only search tracked files:
  grep Foo `hg mani`

.hgignore

@@ -51,4 +51,5 b' syntax: regexp'
 ^\.idea$
 ^\.cache$
 ^\.pytest_cache$
+^venv$
 /__pycache__$

docs/contributing.rst

@@ -33,8 +33,8 b' bash shell::'
 
         hg clone https://kallithea-scm.org/repos/kallithea
         cd kallithea
-        python3 -m venv ../kallithea-venv
-        . ../kallithea-venv/bin/activate
+        python3 -m venv venv
+        . venv/bin/activate
         pip install --upgrade pip setuptools
         pip install --upgrade -e . -r dev_requirements.txt python-ldap python-pam
         kallithea-cli config-create my.ini

docs/installation.rst

@@ -53,8 +53,8 b' repository, use the following commands i'
 
         hg clone https://kallithea-scm.org/repos/kallithea -u stable
         cd kallithea
-        python3 -m venv ../kallithea-venv
-        . ../kallithea-venv/bin/activate
+        python3 -m venv venv
+        . venv/bin/activate
         pip install --upgrade pip setuptools
         pip install --upgrade -e .
         python3 setup.py compile_catalog   # for translation of the UI
@@ -81,6 +81,8 b' problematic when upgrading the system or'
 An additional benefit of virtualenv is that it doesn't require root privileges.
 
 - Don't install as root - install as a dedicated user like ``kallithea``.
+  If necessary, create the top directory for the virtualenv (like
+  ``/srv/kallithea/venv``) as root and assign ownership to the user.
 
 - Create a new virtual environment, for example in ``/srv/kallithea/venv``,
   specifying the right Python binary::

docs/overview.rst

@@ -79,7 +79,7 b' with home in ``/home/kallithea`` and pla'
 
 For experiments, it might be convenient to run everything as yourself and work
 inside a clone of Kallithea, with the ``.ini`` and SQLite database in the root
-of the clone.
+of the clone, and a virtualenv in ``venv``.
 
 
 Python environment