This is similar to \tag, but it should autogenerate a one or two sentence intro on every post that uses it. E.g., something like:
This post is part of a series named “XYZ” where I share XYZ-description.
The XYZ to XYZ-description mapping should probably be done in the config. The actual text blurb ("This post is part of "{series}" where I share {desc}.") should be a template.
The actual indexing of posts can be done via tags.