User documentation for the deposit service

related to swh-deposit#4655

Reasons for this MR:

1. The deposits mechanisms are no longer be limited to the swh-deposit package (see swh-coarnotify),
2. The organization of the current swh-deposit documentation could be improved to match more closely the needs of our user and, most importantly, prospects

Key-points of this MR:

• make a new user oriented documentation for our deposit service (not a documentation on the swh-deposit software)
• following diataxis principles
• avoid content duplication especially for references

Our immediate goal is to leverage this documentation to showcase the deposit service(s) effectively to prospective partners.

Changes

• the new doc will live in swh-docs/users/deposit
• its entrypoints are:
  • a new lvl2 chapter on the landing page under Archive source code called Deposit artefacts and/or metadata leading to the /deposit path
  • a new entry is /user
• the documentation will follow the plan laid out in swh-deposit#4655

Screenshots

A live preview is also available on https://polite-meerkat-9b0f00.netlify.app/user/deposit/

swh-docs landing page changes

deposit services index

docs/deposit/explanations/code-deposit.rst

+What's a code deposit?
+======================
+
+Most of the software source code artifacts present in the SWH Archive are gathered by
+the mean of `loader`_ workers run by the SWH project from source code origins identified
+by `lister`_ workers. This is a pull mechanism: it's the responsibility of the SWH
+project to gather and collect source code artifacts that way.
+
+Alternatively, SWH allows its partners to push source code artifacts and metadata
+directly into the Archive with a push-based mechanism. By using this possibility
+different actors, holding software artifacts or metadata, can preserve their assets
+without having to pass through an intermediate collaborative development platform, which
+is already harvested by SWH (e.g GitHub, GitLab, etc.).
+
+**This mechanism is the code deposit.**
+
+The main idea is the deposit is an authenticated access to an API allowing the user to
+provide source code artifacts -- with metadata -- to be ingested in the SWH Archive. The
+result of that is a `SWHID`_ that can be used to uniquely and persistently identify that
+very piece of source code.
+
+This unique identifier can then be used to `reference the source code
+<https://hal.archives-ouvertes.fr/hal-02446202>`_ (e.g. in a `scientific paper
+<https://www.softwareheritage.org/2020/05/26/citing-software-with-style/>`_) and
+retrieve it using the `vault`_ feature of the SWH Archive platform.
+
+The differences between a piece of code uploaded using the deposit rather than simply
+asking SWH to archive a repository using the `save code now`_ feature are:
+
+- a deposited artifact is provided from one of the SWH partners which is regarded as a
+  trusted authority,
+- a deposited artifact requires metadata properties describing the source code artifact,
+- a deposited artifact has a codemeta_ metadata entry attached to it,
+- a deposited artifact has the same visibility on the SWH Archive than a collected
+  repository,
+- a deposited artifact can be searched with its provided url property on the SWH
+  Archive,
+- the deposit API uses the `SWORD v2`_ API, thus requires some tooling to send deposits
+  to SWH. These tools are provided with this repository.
+
+.. _SWHID: https://docs.softwareheritage.org/devel/swh-model/persistent-identifiers.html#persistent-identifiers
+.. _vault: https://docs.softwareheritage.org/devel/swh-vault/index.html#swh-vault
+.. _save code now: https://archive.softwareheritage.org/save/
+.. _SWORD v2: http://swordapp.org/sword-v2/
\ No newline at end of file