feat: add ADR Process, inspired by the backstage ADR structure

docs/architecture-decisions/adr000-template.md

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+______________________________________________________________________
+
+| id          | title             | description                                                      |
+| ----------- | ----------------- | ---------------------------------------------------------------- |
+| adrs-adr000 | ADR000: \[TITLE\] | Architecture Decision Record (ADR) for \[TITLE\] \[DESCRIPTION\] |
+
+______________________________________________________________________
+
+<!-- These documents have names that are short noun phrases. For example, "ADR001: Deployment on Ruby on Rails 3.0.10" or "ADR009: LDAP for Multitenant Integration" -->
+
+## Context
+
+<!--
+This section describes the forces at play, including technological, political, social, and project local. These forces are probably in tension, and should be called out as such. The language in this section is value-neutral. It is simply describing facts. -->
+
+## Decision
+
+<!-- This section describes our response to these forces. It is stated in full sentences, with active voice. "We will ..." -->
+
+## Consequences
+
+<!-- This section describes the resulting context, after applying the decision. All consequences should be listed here, not just the "positive" ones. A particular decision may have positive, negative, and neutral consequences, but all of them affect the team and project in the future. -->
+
+<!-- This template is taken from a blog post by Michael Nygard http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions -->

docs/architecture-decisions/index.md

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+______________________________________________________________________
+
+| id            | title                               | sidebar_label | description                                     |
+| ------------- | ----------------------------------- | ------------- | ----------------------------------------------- |
+| adrs-overview | Architecture Decision Records (ADR) | Overview      | Overview of Architecture Decision Records (ADR) |
+
+______________________________________________________________________
+
+The architecture decisions made for the Einride Extend public API are kept here. For more information about ADRs, when to write them, and why, please see [this blog post](https://engineering.atspotify.com/2020/04/14/when-should-i-write-an-architecture-decision-record/).
+
+Records are never deleted but can be marked as superseded by new decisions or deprecated.
+
+Records should be stored under the `architecture-decisions` directory.
+
+## Contributing
+
+### Creating an ADR
+
+- Copy `docs/architecture-decisions/adr000-template.md` to`docs/architecture-decisions/adr000-my-decision.md` (my-decision should be descriptive. Do not assign an ADR number.)
+- Fill in the ADR following the guidelines in the template
+- Submit a pull request
+- Address and integrate feedback from the community
+- Eventually, assign a number
+- Merge the pull request
+
+## Superseding an ADR
+
+If an ADR supersedes an older ADR then the status of the older ADR is changed to "superseded by ADR-XXXX", and links to the new ADR.