Add terminology and topics

[sgreenbury]

Searching for metrics of interest from a given country's data can be challenging without knowing how metrics are categorised, structured and defined.

Suggestions for how this could be improved:

• Adding more information or links to background on a given country's census data terminology
• Consider implementing a "category mode" where a given topic's (e.g. "mobility" or "health") metrics could be filtered to show what metrics are available
• Enable the search functionality to work with words that refer to the same topic. For example, some censuses have metrics referring to "cars" while others "vehicles" (or another example: "tenure" and "housing"). The LLM embedding functionality being developed as part of popgetter would help towards this, though this solution would require a server for the database of embeddings.