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/deposit.rst

@@ -48,3 +48,9 @@ Is it useful for me?
 Source code is fragile; it can disappear. It is important to note that software source code has an essential role in research and should be archived properly, alongside data and publications. Software that was built for research as part of the open science ecosystem should be archived, referenced, described and cited.
 
 When depositing in Software Heritage you can describe a software artifact properly with specific metadata properties and it will be safely saved in the universal software archive.
+
+Also, as a metadata producer you can attach to an existing entry in the archive all the
+TODO
+
+To understand why metadata is so important to us read
+:ref:`Why do we need metadata? <deposit-why-metadata>`.
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