Add policy around package versioning

To avoid confusions, create a document to outline the policies currently
used.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/policies/application_versionig.md

++++
+title = "Package versioning"
+weight = 100
+outputs = [ "html", "pdf-in",]
+date = "2024-10-31"
++++
+
+Apertis is an source distribution based on Debian. As such it follows the same rules as Debian for
+the versioning of the packages and applications.
+
+However, besides the general guidelines from upstream, it is useful to clarify different use cases
+in order to provide clear policies.
+
+For this reason, the current document describe different scenarios and how to deal with them.
+
+## Update for Debian package
+
+This is the most common case, and the simplest one. Apertis adds the suffix `+apertis` and an Apertis
+revision starting form `0`. Apertis specific changes bumps the Apertis revision.
+
+**Example:**
+
+Starting point:
+- Debian: dash 0.5.12-2
+- Apertis: dash 0.5.12-2+apertis0 (v2025pre)
+
+After update:
+- Apertis: dash 0.5.12-2+apertis1 (v2025pre)
+
+## Update for Apertis native packages
+
+In this case, the upstream repo for the project is in Apertis, so a versioning scheme should be used
+which aligns with the release cycle and makes it easier to plan backports. Taking this into account
+Apertis uses a follow the `major.minor.patch` schema with `major=0`, `minor=release` and `patch=version`.
+This election aims to make it easier to backports changes and diverge to adapt to different Apertis
+release.
+
+In case that the update is planned to be backported the version should only increment the patch number,
+even in the case of new release.
+
+**Example**
+
+Starting point:
+- Apertis apertis-dev-tools 0.2024.15 (v2025pre)
+- Apertis apertis-dev-tools 0.2024.15 (v2024)
+
+After update that is planned to be backported:
+- Apertis apertis-dev-tools 0.2024.16 (v2025pre)
+- Apertis apertis-dev-tools 0.2024.16 (v2024)
+
+However, in case of update not planned to be backported, the minor number should increase and match the
+release number.
+
+**Example**
+
+Starting point:
+- Apertis apertis-dev-tools 0.2024.15 (v2025pre)
+- Apertis apertis-dev-tools 0.2024.15 (v2024)
+
+After update that is **not** planned to be backported:
+- Apertis apertis-dev-tools 0.2025.0 (v2025pre)
+- Apertis apertis-dev-tools 0.2024.15 (v2024)
+
+## Backport of Debian update for Debian package
+
+Due to the different release cycles between Debian and Apertis it is possible that the same version of the
+package is found in multiple Apertis release. In this case the backport is straight forward as the exact
+same version is applied in older releases.
+
+However, due to the needs of each Apertis release, in some occasions, the backport is not straight forward
+as the same Debian version of the package requires different version in different Apertis releases. In this
+case a Apertis release suffix should be added.
+
+**Example:**
+
+Starting point:
+- Debian: dash 0.5.12-1
+- Apertis: dash 0.5.12-1+apertis1 (v2025pre)
+- Apertis: dash 0.5.12-1+apertis0 (v2024)
+
+After update:
+- Debian: dash 0.5.12-2
+- Apertis: dash 0.5.12-2+apertis0 (v2025pre)
+- Apertis: dash 0.5.12-2+apertis0~v2024 (v2024)
+
+## Backport of Apertis update for Debian package
+
+In case of Apertis specific updates, there are different scenarios. The first one and simpler is that the
+older release was matching the same version, in which case the exact same version will be applied. The second
+one, is when there are different Debian version. This is also simple, in the sense that the update needs to be
+applied on top of the older version and the Apertis revision needs to be increased.
+
+**Example:**
+
+Starting point:
+- Apertis: dash 0.5.12-2+apertis0 (v2025pre)
+- Apertis: dash 0.5.12-1+apertis0 (v2024)
+
+After update:
+- Apertis: dash 0.5.12-2+apertis1 (v2025pre)
+- Apertis: dash 0.5.12-1+apertis1 (v2024)
+
+The third case is when there version in the older release is the same Debian version but an older Apertis
+revision. In this case we cannot increment the Apertis suffix in the older release since that version already
+exits, so and addition suffix needs to be added.
+
+**Example:**
+
+Starting point:
+- Apertis: dash 0.5.12-2+apertis1 (v2025pre)
+- Apertis: dash 0.5.12-2+apertis0 (v2024)
+
+After update:
+- Apertis: dash 0.5.12-2+apertis2 (v2025pre)
+- Apertis: dash 0.5.12-1+apertis1+v2024 (v2024)
+
+## Reference
+- [Debian policy for package versioning](https://www.debian.org/doc/debian-policy/ch-controlfields.html#version):
+it contains a comprehensive explanation of how the versions work.
+- [Debian Go policy for package versioning](https://go-team.pages.debian.net/packaging.html#_version_numbers):
+it provides a guide for generating a version when upstream does not use version numbers (for instance, it's
+common for Go libraries to have no version numbers in the traditional sense).
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