Update docs frontpage to show mapping metadata and examples at a glance

[matentzn]

Resolves #392

This PR updates the front page to show at a glance the mapping level metadata fields, and a better view of a SSSOM example table.

• docs/ have been added/updated if necessary
• make test has been run locally
• tests have been added/updated (if applicable)
• CHANGELOG.md has been updated.

[cthoyt]

I want to review this but the make gendoc command gives the following error:

$ make gendoc
cp -rf src/docs/* docs ; \
	poetry run gen-doc -d docs src/sssom_schema/schema/sssom_schema.yaml --template-directory src/doc-templates --index-name linkml-index
Command not found: gen-doc
make: *** [gendoc] Error 1

I tried a couple other things but I am not very good with makefiles and it's not really clear what to do in what order. make install also had issues, I'm assuming due to issues with poetry (it's time to switch to uv ;))

[matentzn]

Poetry install?

[matentzn]

Here is the generated markdown:

https://www.dropbox.com/scl/fi/bqf5puioqogqvipzs2ev3/sssom_index_updated.md?rlkey=h59uwwkuursys8wf4hcvndort&dl=0

Anything else you want to see?

[cthoyt]

This gets the job done nicely, thanks

[jamesamcl]

The content LGTM, I don't know enough about the jinjinate stuff to comment but I'm sure it's fine