Add make linkcheck step to build-conda and build-pip... [#29]

...to check external links

Note that this blocks on the various PRs attached to
nextstrain/docs.nextstrain.org/#14 — those PRs should be merged before
this one.

.github/workflows/docs-ci.yaml

@@ -68,6 +68,9 @@ jobs:
       - run: make ${{ inputs.make-target }}
         working-directory: ${{ inputs.docs-directory }}
 
+      - run: make linkcheck
+        working-directory: ${{ inputs.docs-directory }}
+
   build-pip:
     if: inputs.pip-install-target != ''
     runs-on: ubuntu-latest
@@ -82,3 +85,6 @@ jobs:
 
       - run: make ${{ inputs.make-target }}
         working-directory: ${{ inputs.docs-directory }}
+
+      - run: make linkcheck
+        working-directory: ${{ inputs.docs-directory }}