Last modification date

As it is expensive to compute embeddings, when extracting content from a website, it could be interesting to store the vectors and reuse them next time we need them.
If so, it could be useful to have a last modification date for each web page in order to update the vectors when needed (and when needed only).

No files, tests, or entry points are identified in the issue. Clarify how page modification dates and stored vectors should be represented, how cached vectors are invalidated, and how the behavior will be verified.

Written by the indexing model from the issue text.