Tool usage page: fix time range claims and add utility

Part of #1175. Page: docs/connector-gateway/tool-usage.mdx

Problems

The time range claim is misleading (lines 12-13). The page implies the time range controls everything on the screen. The options are Last 24h, Last 7d, and Last 30d, with 24h the default. They drive the tiles and the tables, but the Tool calls trend chart always shows Last 3 months, whatever range is selected.
The page doesn't help the reader answer anything. It lists what's on screen, but not what an admin can learn from it, such as which connectors are unused, where adoption is growing, or which tools are called most. It doesn't mention drill-down either. Connector names in the tables aren't links, and per-user detail is only on each connector's Activity tab.
Troubleshooting is mixed into the body (lines 20-22), and it leaves out the most likely cause of an empty screen: the metering prerequisite tracked in Connector Gateway: add quickstart, user, telemetry, troubleshooting #1179.
Minor:
- The console title is Tool Usage (title case). Match it when bolding the UI label.
- Line 15 says "integrations". Use "connectors".

List the time range options and the default, and state that the chart's window is fixed at 3 months
Document the tiles (Tool calls, Active connectors, Top connector) and the table columns (By connector: Connector, Calls, Share; By tool: Tool, Connector, Calls, Share)
Frame the page around two or three admin questions it answers
Link to the connector Activity tab for per-user and per-call detail
Mention that users see their own usage on the Usage tab in the User experience
Move lines 20-22 into a Troubleshooting section, and add the metering prerequisite once Connector Gateway: add quickstart, user, telemetry, troubleshooting #1179 confirms it