Update CoWI webpage with Handbook links

Update the following College of Water Informatics pages with revised links to the Handbook

- [ ] Data Tool Kit - Data Management Handbook; [Plan & Prepare section](https://www.waterboards.ca.gov/resources/oima/cowi/data_management_handbook.html#plan_prepare)
- [ ] Replace "If you are unsure where to start, try using this [Water Board Data Management Plan Template](https://docs.google.com/document/d/1rnJp8NQ20uZ8r7bf12wcAEJzIfDi14iyc2B3v3xReFc/edit?usp=sharing)!" with "If you are unsure where to start, check out the Water Board Equity Data Handbook's [Preparation section](https://cawaterboarddatacenter.github.io/equity-data-handbook/prep.html)

- [ ] Data Tool Kit - Open Data Handbook; add an open data portals section and list the following portals
- [ ] [California Open Data Portal](https://data.ca.gov/)
- [ ] [CA State Geoportal](https://gis.data.ca.gov/)
- [ ] [Water Board's Data and Databases](https://www.waterboards.ca.gov/resources/data_databases/)
- [ ] [National Water Quality Portal](https://www.waterqualitydata.us/)

Start by locating the Data Tool Kit's Data Management Handbook and Open Data Handbook pages in the repository, then compare their current links and sections with the issue checklist. Done means the outdated template reference is replaced, the open data portals section lists all four requested portals, and every checklist item is satisfied.

Written by the indexing model from the issue text.