[Content Gap] Add keyword taxonomy guidance for federal dataset inventories

## What needs to change
The site has no guidance on how to develop or maintain a keyword taxonomy for a dataset inventory. Federal data practitioners building or improving agency data catalogs frequently need to make decisions about controlled vocabularies, keyword selection, and how tagging connects to discoverability. This is a practical operational gap that fits squarely in the Data Management & Governance category.

## What to create
A new summary resource (or a curated set of linked resources) in `pages/_summaries/` covering keyword taxonomy development for federal dataset inventories. The goal is not to write original guidance but to surface and link to authoritative resources that already exist, with a short editorial framing note explaining their relevance.

## Suggested resources to research and verify

**DCAT-US Schema field guidance:**
- The `keyword` and `theme` fields in the DCAT-US Schema are the most directly relevant. The `theme` field recommends use of controlled vocabularies. Document what those recommended vocabularies are and link to them from the summary.
- Verify current guidance at https://resources.data.gov/resources/dcat-us/

**OMB and federal catalog standards:**
- Check whether OMB OFCIO has issued any guidance on metadata tagging or keyword standards for data.gov submissions beyond DCAT-US.
- Review data.gov publisher documentation for any keyword or theme requirements imposed at catalog ingestion.

**Federal data community practice:**
- Check the CDO Council resources at https://www.cdo.gov for any published guidance on data inventory tagging or metadata governance.
- Check whether the Federal Data Strategy action plan or supporting materials address keyword taxonomy as part of data inventory requirements under the OPEN Government Data Act (P.L. 115-435).

**General best practice references:**
- NIST or Dublin Core controlled vocabulary guidance relevant to government datasets.
- Any W3C DCAT documentation on the `dcat:keyword` vs `dcat:theme` distinction that would help practitioners understand the difference and when to use each.

## How to add it
Add one or more summary files in `pages/_summaries/` using the standard front matter format. Suggested keywords: `data-management`, `metadata`, `data-inventory`, `data-catalog`. Category: `data-management-governance`.

## How to verify
Visit https://resources.data.gov/categories/data-management-governance and confirm the new resource(s) appear. Also visit https://resources.data.gov/keywords/metadata/ to confirm the entry surfaces there.