cellfinder-core and cellfinder-napari merge update

[willGraham01]

See brainglobe/BrainGlobe#46

Updates the website with blog posts, changelog, and user docs for the cellfinder core and napari plugins merging.

• The November roadmap now has outdated information and links, but this is deliberately left like this since it's a reflection of the state of BrainGlobe at that time
• Blog post added
• Changelog added
• Install recommendations updated

[adamltyson]

Looks good, thanks @willGraham01. I'm ok for it to be merged as-is, but I've raised #129 because I think the current status of the cli being included within the cellfinder section is confusing. This will be more confusing when we rename the CLI (which I think we should do soon).

[willGraham01]

@adamltyson I've attempted to disentangle the cellfinder package docs from the now-new brainmapper docs - mostly involves a lot of moving things around and updating some names, but you might want to double check:

• The brainmapper tutorial
• The brainmapper documentation
• The remaining cellfinder information

[adamltyson]

Thanks for tackling the cellfinder->brainmapper issue @willGraham01, I wasn't looking forward to doing it myself!

I've left a few comments. TBH I think this naming migration will drag on for a while as we keep finding outdated references. Feel free to raise issues rather than tackle everything!