Add open access APIs in TfL section

[dch82]

I'm not sure as to the current state of Unified API but on the TFL API portal, it reads:

This API document is a legacy from our last portal. All the other APIs are bundled here as a single ungrouped OpenAPI document. You are advised to use the other APIs for documentation.

It also lists several other APIs. AFAIK all the endpoints on the Unified API can be accessed through these separate APIs, as well as not needing API keys to access. I think these should replace the Unified API on the catalogue.