Make use of the common adr template in the .github repo (#1599)

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## Overview

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To unify the ADR structure we added a template in the .github repo. This
PR adds a helpful command to generate new ADRs from that template.

Example:     https://asciinema.org/a/56X6g1bA9NpWq0FxW8wDMRJ3g

Closes #1415 

## Checklist

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- [x] New and updated code has appropriate documentation
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- [x] Linked issues closed with keywords

---------

Co-authored-by: Evan Forbes <[email protected]>
Co-authored-by: Rootul P <[email protected]>

Makefile

@@ -127,3 +127,9 @@ test-cover:
 	@echo "--> Generating coverage.txt"
 	@export VERSION=$(VERSION); bash -x scripts/test_cover.sh
 .PHONY: test-cover
+
+## adr-gen: Download the ADR template from the celestiaorg/.github repo. Ex. `make adr-gen NUM=016 TITLE=my-adr-title`
+adr-gen:
+	@echo "--> Generating ADR template"
+	@curl -sSL https://raw.githubusercontent.com/celestiaorg/.github/main/adr-template.md > docs/architecture/adr-$(NUM)-$(TITLE).md
+.PHONY: adr-gen