docs [installation]: Add installation instructions

Add some basic installation instructions via pip or conda.

instructions/installation.md

+# Installation instructions (bob >= 12.0.0)
+
+To install a bob package in order to use its commands and/or python API, you can either
+use `conda` or `pip`.
+
+## Installing a bob package with pip
+
+### Installing a stable version
+
+To install a stable (released) package with pip, you can use the ones deployed on pypi.
+For example to install the latest stable `bob.bio.face`:
+
+``` sh
+pip install bob.bio.face
+pip list | grep bob.bio.face
+```
+
+should output (with any version number but a beta):
+
+``` text
+bob.bio.face      7.1.2
+```
+
+If you want to install a beta of a package via pip, you will need to specify our own
+registry as an additional `index-url`:
+
+``` sh
+pip install --extra-index-url https://gitlab.idiap.ch/api/v4/groups/373/-/packages/pypi/simple bob.bio.face
+pip list | grep bob.bio.face
+```
+
+should output (with any beta version number):
+
+``` text
+bob.bio.face      7.1.3b0
+```
+> **NOTE:**
+>
+> If you regularly need to give this extra `index-url`, you can add it to your
+> `~/.config/pip/pip.conf`:
+>
+> ``` text
+> [global]
+> trusted-host = gitlab.idiap.ch
+> extra-index-url = https://gitlab.idiap.ch/api/v4/groups/373/-/packages/pypi/simple
+> ```
+
+## Installing a bob package with conda
+
+
+> **NOTE:**
+>
+> The following commands use `mamba` as it operates much faster than the regular conda's
+> `create` and `install` commands.
+>
+> If you don't have mamba installed, you can replace all `mamba xxx` commands by
+> `conda xxx`.
+
+### Installing a stable version
+
+To create an environment containing a stable (released) bob package with conda, you can
+use the following command (assuming you have
+[a working conda installation](https://docs.conda.io/projects/conda/en/latest/user-guide/install/index.html#regular-installation)):
+
+``` sh
+mamba create -n bob_face_stable --override-channels -c https://www.idiap.ch/software/bob/conda -c conda-forge -c defaults bob.bio.face
+conda activate bob_face_stable
+conda list bob.bio.face
+```
+
+should output (with any version number but a beta):
+
+``` text
+# packages in environment at /your/path/to/miniconda3/envs/bob_face_stable:
+#
+# Name            Version              Build  Channel
+bob.bio.face      7.1.2       py39hb6b4a9b_0  https://www.idiap.ch/software/bob/conda
+```
+
+### Installing a beta version
+
+To create a conda environment containing a beta version of a bob package, you can use
+the same method but adding the correct channel:
+
+``` sh
+mamba create -n bob_face_beta --override-channels -c https://www.idiap.ch/software/bob/conda/label/beta -c conda-forge -c defaults bob.bio.face
+conda activate bob_face_beta
+conda list bob.bio.face
+```
+
+should output (with any beta version):
+
+``` text
+# packages in environment at /your/path/to/miniconda3/envs/bob_face_beta:
+#
+# Name            Version              Build  Channel
+bob.bio.face      7.1.3b0      pyhe1939b0_36  https://www.idiap.ch/software/bob/conda/label/beta
+```
+
+> **NOTE:**
+>
+> You can also install a bob package in an existing conda environment:
+>
+> ``` sh
+> conda activate bob_face_beta
+> mamba install --override-channels -c https://www.idiap.ch/software/bob/conda/label/beta -c conda-forge -c defaults bob.pad.face
+> ```