Add research context documentation as vignette and blog

[milanwiedemann]

@LFISHER7 suggested we might want to add documentation explaining why researchers would use the opencodecounts R package and code usage data more generally. Currently, this context exists in the draft manuscript but isn't easily accessible to potential users.

Proposed solution

• Create a vignette explaining the research rationale for using code usage data
• Adapt this content as a blog post for https://www.bennett.ox.ac.uk/blog/
• Link to existing blog series (e.g., https://www.bennett.ox.ac.uk/blog/series/clinical-codes/) and other documentation sections where appropriate

Context

• This would complement existing technical documentation by providing the research context upfront
• Content can largely draw from the manuscript introduction but might be more accessible as a standalone piece
• Will help users understand when/why they might choose opencodecounts vs other tools (e.g., [Shrimp SNOMED browser](https://ontoserver.csiro.au/shrimp/) and The NHS Digital SNOMED CT Browser)

[atamborska]

I have drafted a blog post here: https://docs.google.com/document/d/1F8A_qTmhqm9gSPSquGhUmRYvr7zYFR8FFRbRqbFaSl8/edit?tab=t.0