Add SPEC tags rendering

[stefanv]

Requires some tags in the specs themselves, e.g.:

diff --git a/spec-0000/index.md b/spec-0000/index.md
index f5c0cd5..91b3191 100644
--- a/spec-0000/index.md
+++ b/spec-0000/index.md
@@ -23,6 +23,8 @@ endorsed-by:
   - scipy
   - xarray
   - zarr
+tags:
+  - maintenance
 ---

 ## Description
diff --git a/spec-0005/index.md b/spec-0005/index.md
index 3815e49..21f6801 100644
--- a/spec-0005/index.md
+++ b/spec-0005/index.md
@@ -10,6 +10,8 @@ author:
   - "Brigitta Sipőcz <brigitta.sipocz@gmail.com>"
 discussion: https://discuss.scientific-python.org/t/spec-5-ci-best-practices/507
 endorsed-by:
+tags:
+  - best-practice
 ---

 ## Description

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