Adding Spack documentation to Sphinx and added changelog entry

    when building to precisely see how a GekkoFS instance has been configured.

- Added (parallel) append support for consecutive writes with file descriptor opened

  with `O_APPEND` ([!164](https://storage.bsc.es/gitlab/hpc/gekkofs/-/merge_requests/164)).

- Added support for Spack so that it can be used to install

  GekkoFS ([!137](https://storage.bsc.es/gitlab/hpc/gekkofs/-/merge_requests/137)).

### Changed

- `AGIOS <https://github.com/francielizanon/agios>`_ (commit c26a654 or

  newer) to enable the :code:`GekkoFWD` I/O forwarding mode.

- `PARALLAX` There are two different metadata backends in GekkoFS. The default one uses `rocksdb`, however an alternative based on `PARALLAX` from `FORTH` 

is available. To enable it, use the `-DGKFS_ENABLE_PARALLAX:BOOL=ON` option, you can also disable `rocksdb` with `-DGKFS_ENABLE_ROCKSDB:BOOL=OFF`.

  Once it is enabled, `--dbbackend` option will be functional.

- :code:`PARALLAX` There are two different metadata backends in GekkoFS. The default one uses :code:`rocksdb`, however

an alternative based on :code:`PARALLAX` from :code:`FORTH` is available. To enable it, use the

:code:`-DGKFS_ENABLE_PARALLAX:BOOL=ON` option, you can also disable :code:`rocksdb` with

:code:`-DGKFS_ENABLE_ROCKSDB:BOOL=OFF`. Once it is enabled, :code:`--dbbackend` option will be functional.

.. _step_by_step_installation:

- GekkoFS daemon (server): :code:`${GKFS_INSTALL_PATH}/bin/gkfs_daemon`

- GekkoFS client interception library: :code:`${GKFS_INSTALL_PATH}/lib/libgkfs_intercept.so`

Install GekkoFS via Spack (alternative)

---------------------

Spack is a package manager for supercomputers and Linux. It makes it easy to install scientific software for regular

users. Spack is another method to install GekkoFS where Spack handles all the dependencies and setting up the environment.

Install Spack

=========================

First, install Spack. You can find the instructions here: https://spack.readthedocs.io/en/latest/getting_started.html

    .. code-block:: console

        git clone https://github.com/spack/spack.git

        . spack/share/spack/setup-env.sh

.. attention::

    Note that the second line needs to be executed every time you open a new terminal. It sets up the environment for Spack

and the corresponding environment variables, e.g., $PATH.

Install GekkoFS with Spack

=========================

To install GekkoFS with Spack, the GekkoFS repository needs to be added to Spack as it is not part of the official Spack

repository.

    .. code-block:: console

        spack repo add gekkofs/scripts/spack

When added, the GekkoFS package is available. Its installation variants and options can be checked via:

    .. code-block:: console

        spack info gekkofs

Then install GekkoFS with Spack:

    .. code-block:: console

        spack install gekkofs

        # for installing tests dependencies and running tests

        spack install -v --test=root gekkofs

Finally, GekkoFS is loaded into the currently used environment:

    .. code-block:: console

        spack load gekkofs

This installs the latest release version including its required Git submodules. The installation directory is

:code:`$SPACK_ROOT/opt/spack/linux-<arch>/<compiler>/<version>/gekkofs-<version>`. The GekkoFS daemon (:code:`gkfs_daemon`) is

located in the :code:`bin` directory and the GekkoFS client (:code:`libgkfs_intercept.so`) is located in the :code:`lib` directory.

Note that loading the environment adds the GekkoFS daemon to the `$PATH` environment variable. Therefore, the GekkoFS

daemon is started by running :code:`gkfs_daemon`. Loading GekkoFS in Spack further provides the :code:`$GKFS_CLIENT` environment

variable pointing to the interception library.

Therefore, the following commands can be run to use GekkoFS:

        .. code-block:: console

            # Consult `-h` or the Readme for further options

            gkfs_daemon -r /tmp/gkfs_rootdir -m /tmp/gkfs_mountdir &

            LD_PRELOAD=$GKFS_CLIENT ls -l /tmp/gkfs_mountdir

            LD_PRELOAD=$GKFS_CLIENT touch /tmp/gkfs_mountdir/foo

            LD_PRELOAD=$GKFS_CLIENT ls -l /tmp/gkfs_mountdir

When done using GekkoFS, unload it from the environment:

    .. code-block:: console

        spack unload gekkofs

Miscellaneous

=========================

Use GekkoFS's latest version (master branch) with Spack:

        .. code-block:: console

            spack install gekkofs@latest

Use a specific compiler on your system, e.g., gcc-11.2.0:

        .. code-block:: console

            spack install gekkofs@latest%gcc@11.2.0

 No newline at end of file

### Install Spack

First, install Spack. You can find the instructions here: https://spack.readthedocs.io/en/latest/getting_started.html

First, install Spack. You can find the instructions [here](https://spack.readthedocs.io/en/latest/getting_started.html)

```bash

git clone https://github.com/spack/spack.git