doc: generate README with docgen tool (#251)

Introduces a docgen tool for templated documentation generation, and uses it
to generate the README.

At the moment this change makes minimal difference to generating it with
embedmd. The difference is that docgen opens up the possibility to generate
documentation with more elaborate templating. The specific use case currently
in mind is including an adopters list that's kept in sync with the third-party
packages file.

Updates #101

script/doc

@@ -1,3 +1,9 @@
 #!/bin/bash -ex
 
+# Generate some specific files with docgen.
+go install ./internal/cmd/docgen
+
+docgen -type readme -output README.md
+
+# Process simple file embeddings with embedmd.
 find . -name '*.md' | xargs embedmd -w