feat(assistants): Add "Discover Assistant" Page

[knajjars]

Screen.Recording.2024-06-17.at.6.09.28.PM.mov

Closes OS-2132

Note: the "Discover" icon is not the one in the designs due to missing default styling. I already requested the asset and will update once we have it

AI Description

This PR adds a new feature to the Agents interface, allowing users to discover assistants.

Summary

The main changes include:

• A new DiscoverAgentCard component to display agent cards with names, descriptions, and a "Try now" button.
• A new Discover option in the AgentsSidePanel component, allowing users to navigate to the discovery page.
• A new Discover page (/agents/discover) that lists available agents and allows users to search for specific agents.
• A new background image for the agent discovery page.

Details

• The DiscoverAgentCard component in DiscoverAgentCard.tsx renders a card for an agent with the agent's name, description, and a "Try now" button. The card also includes a logo or the first letter of the agent's name, with a color based on the agent's ID.
• The AgentsSidePanel component in AgentsSidePanel.tsx now includes a Discover option in the navigation items, allowing users to navigate to the discovery page.
• The AgentsSidePanel component now conditionally renders the navigation items based on the isAgentsSidePanelOpen flag. When the panel is open, the navigation items are rendered as buttons; otherwise, they are rendered as icon buttons with tooltips.
• The AgentsList component in AgentsList.tsx now uses the truncate class for the "Most recent" text to prevent text overflow.
• A new Discover page is added in discover/index.tsx, which lists available agents and allows users to search for specific agents. The page uses the DiscoverAgentCard component to display each agent.
• A new background image is added in cellBackground.svg for the agent discovery page.

[abimacarmes]

Do we want to update component names to match the updated "Assistant" language?