docs: Explain GH tag naming

docs/source/maintenance/github-repo.md

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+# GitHub Repository Maintenance
+
+Just a few things in here which go over some standards in the repo.
+
+## Tags
+
+All tags have a two-digit number prepended to their name, which enforces a specific ordering, as well as groups them.
+This number also determines their colour.
+
+| First Digit | Meaning                                                                           | Colour            |
+| ----------- | --------------------------------------------------------------------------------- | ----------------- |
+| 0           | Issue classification - bug, enhancement, etc                                      | Red               |
+| 1           | Meta-tags - duplicate, enhancement, etc                                           | Green             |
+| 2 or 3      | Software-related tags                                                             | Teal              |
+| 4           | Task specific payload tags                                                        | Yellow            |
+| 5           | Subsystem classification (if applicable) - this one's less strict than the others | Dark blue         |
+| 6           | Build system - Nix, CMake, etc                                                    | Purple            |
+| 7           | Hardware specific issues                                                          | Light/pastel teal |