Align DSR Split View to SLDS

Interaction Updates

• [Accessibility] Add instructions to the first item in the list to indicate Shift + Tab will take the user back to the list after selecting an item.
• Do not change the information on the right unless the user clicks on an item or presses enter
• The user should be able to navigate the items using the up/down arrow keys which should move a 1px blue border up/down
• The selected item should only have a 1px blue border if the use is focused on that item
• If the user is focused on another item the selected item should be indicated by the 4px blue box shadow
• Allow the Split View to be collapsed/expanded by clicking on the small triangle (triangle should change direction based on collapsed/expanded state...see SLDS)

Visual Updates

• Use <header class=slds-split-view__header"> instead of <div class="slds-page-header slds-page-header_object-home slds-split-view__header slds-has-bottom-magnet"> and update code in <header> to match the SLDS component
• Remove slds-scrollable_none from <div class="slds-grid slds-grid_vertical slds-scrollable_none"> and update code in <div> to match the SLDS component
• Make sure the Split View doesn't change width when selecting different items (when I had the DSR window smaller the width shifted as I used the arrow keys/clicked on an item)\

In general, the markup should match https://www.lightningdesignsystem.com/components/split-view/.

Start with the DSR Split View implementation and compare its markup and behavior with the SLDS Split View documentation linked in the issue. Check keyboard navigation, focus styling, collapse behavior, and width stability against each listed interaction and visual requirement; done means the component matches SLDS and all stated interactions work consistently.