Adds LLM matrix for OSS models

[benironside]

Fixes #6396 — adds a second matrix to the LLM performance matrix page that describes the performance of open-source models on our tasks. Also makes a minor addition to the bring your own LLM guide, linking to the performance matrix since it now includes info about open source models.

Previews:

• LLM Performance matrix (ESS) (Serverless)
• BYO LLM setup guide (ESS) (Serverless)

[github-actions]

A documentation preview will be available soon.

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[mergify]

This pull request does not have a backport label. Could you fix it @benironside? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.