Detect wrong or missing paths

[ErikMHP]

Describe the solution you'd like
UbCode should detect and report invalid or missing file paths used in directives e.g.:
• .. image:: directives
• .. include:: statements
• .. figure:: ...
• .. csv-table:: ...
UbCode should show diagnostics for missing or incorrect paths for directives at edit, and underline the issue in directly in the .rst file.

[ubmarco]

Moving this to January. An initial PR exists, but we need improvements in our caching system. When an image or figure is changed, we need to invalidate the cache for this and trigger a re-indexing.