Add "Client Resources" feature to table of contents and main Performance page

[david-poindexter]

Description of documentation change

We need to add a table of contents item to surface new documentation on "Client Resources" from @bdukes via #521. We should also update the main "Performance" landing page to add a link to the sub-page, following a model similar to that shown in the "Getting Started" area of dnndocs.com.

Existing documentation URL (if applicable)

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Existing documentation section (if applicable)

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Proposed area for new documentation (if applicable)

Features > Performance > Client Resources

Screenshots

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Additional context

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