Doc: add pyenv documentation

hugo/content/dev/pyenv.md

++++
+title = "Pyenv"
+weight = 80
++++
+
+[Pyenv](https://github.com/pyenv/pyenv) is a platform to install and manage
+different versions of Python, isolated from the default Python version.
+Its advantage compared to the other options, like Anaconda, is its simplicity and smallness.
+Currently, to build BornAgain libraries for different versions of Python,
+we use the `pyenv` platform.
+
+# Installing `pyenv` platform
+
+#### Linux
+
+Obtain the installer from <https://github.com/pyenv/pyenv-installer>.
+To install `pyenv`, use
+```
+curl https://pyenv.run | bash
+```
+By default, `pyenv` is installed in `$HOME/.pyenv`.
+
+The dependencies must be installed before building Python;
+see <https://github.com/pyenv/pyenv/wiki#suggested-build-environment>:
+
+```
+sudo apt install build-essential libssl-dev zlib1g-dev \
+libbz2-dev libreadline-dev libsqlite3-dev curl \
+libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev
+```
+
+#### MacOS
+
+Install `pyenv` via `homebrew`:
+```
+brew install pyenv
+```
+
+The rest of the `pyenv` instructions are like those for Linux.
+
+
+#### Windows
+
+On MS-Windows, [pyenv-win](https://github.com/pyenv-win) should be used.
+
+The available Python versions are distributed as _binary_, therefore
+one must choose the version from a given list.
+
+The Python environments are installed by default under `%USERPROFILE%\.pyenv\pyenv-win\versions`.
+
+To obtain the `pyenv` root, enter the following in `cmd`:
+```
+echo %PYENV_ROOT%
+```
+
+
+#### Using `pyenv`
+
+**Obtain a list of available Python versions**
+```
+pyenv install --list
+```
+
+**Obtain a list of available Python versions**
+```
+pyenv install --list
+```
+
+**Install a Python version**
+To install a certain version, say 3.10, use:
+```
+pyenv install 3.10
+```
+
+**List installed versions**
+```
+pyenv versions
+```
+
+**Obtain the `pyenv` root folder where all versions are stored**
+```
+pyenv root
+```
+**Obtain the path to the current version of Python in pyenv**
+```
+pyenv which python3
+```
+
+**Install Python packages for a certain version of Python**
+
+If the path to the Python executable is, for instance, `$HOME/.pyenv/versions/3.10.12/bin/python3`,
+then to install `numpy` for this specific Python version, one should use
+```
+$HOME/.pyenv/versions/3.10.12/bin/python3 -m pip install numpy
+```
+
+#### Build BornAgain with a specific Python version
+
+To build BornAgain with a specific Python version, the path to the Python platform should be included in `CMAKE_PREFIX_PATH` variable;
+for instance, if Python 3.10.12 is installed in `$HOME/.pyenv/versions/3.10.12` folder, then `CMAKE_PREFIX_PATH` should include this path.