docs(adr): add ADRs

They have been removed from the docs:
NordicSemiconductor/asset-tracker-cloud-docs@1db3320

Author: coderbyheart

adr/002-use-npm-package-registry.md

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+# ADR 002: Use NPM registry
+
+In March 2020,
+[GitHub acquired NPM](https://github.blog/2020-03-16-npm-is-joining-github/) and
+it will be kept operational as a registry for public JavaScript packages. So, as
+of now, the packages can be moved back to NPM.
+
+This supersedes
+[ADR 001: Use GitHub package registry](https://github.com/NordicSemiconductor/asset-tracker-cloud-docs/blob/d14e0e0ec692bea5e3b1ea0b50b359c0dc8c0c4f/docs/adr/001-use-github-package-registry.md).

adr/003-use-saga-as-the-main-branch.md

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+# ADR 003: Use saga as the name for the main branch
+
+Historically, Git and other software use terms such as `master/slave`,
+`whitelist/blacklist`, which are based on racial concepts. Their continued use
+maintains the racial stereotypes they depict. Better alternatives in meaning and
+technical correctness exist, like `leader/follower`, `blocklist/allowlist`.
+
+In the Nordic mythology, a `saga` is a long, continuous recollection of stories
+about the history of humans, legends, and Gods. The term `saga` reflects very
+well what happens in a Git repository. Changes happen in branches (some teams
+tie them to _User Stories_, which are sometimes directly or loosely coupled to
+the main branch). Once the changes are finalized, they get added to the main
+branch, or get appended in the case of a rebase. The mental model of a big book
+of stories fits this process very well.
+
+Therefore, the main branch in this project is named `saga`. `master` must not be
+used.

adr/004-move-to-nordicsemiconductor-github-organization.md

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+# ADR 004: Move to Nordic Semiconductor GitHub organization
+
+In February 2021, the project was moved from the `Bifravst` GitHub organization
+to the official Nordic Semiconductor GitHub organization
+(`Nordic Semiconductor`). This move was caused by the
+[promotion of the Bifravst project into an official Nordic Semiconductor open-source project](https://github.com/bifravst/bifravst/issues/56)
+with the release of the firmware in
+[nRF Connect SDK](https://github.com/nrfconnect/sdk-nrf) as
+[nRF9160: Asset Tracker v2](https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.5.0/nrf/applications/asset_tracker_v2/README.html)
+application.
+
+The project is renamed to _nRF Asset Tracker_.

adr/005-clean-gitignore.md

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+# ADR 005: Clean `.gitignore` file
+
+A `.gitignore` file in a project must only cover the artifacts caused by the
+contained source code and not those caused by the personal choice of editor or
+the environment of a developer.
+
+This is explained in detail
+[here](https://github.com/coderbyheart/first-principles/issues/30).

adr/README.md

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+# Architecture decision records
+
+This folder contains the architecture decision records (ADRs) for this project.
+
+To know more about ADRs, see
+[Documenting architecture decisions](http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions)
+and the video on
+[Communicating and documenting architectural decisions](https://www.youtube.com/watch?v=rwfXkSjFhzc).