Docs - add information about the SPFx Toolkit extension settings

[Saurabh7019]

SPFx Toolkit has the following VS Code extension settings (visible in VS Code Settings UI under Extensions → SharePoint Framework Toolkit):

1. nodeVersionManager - Choose your preferred Node.js version manager. Choose between nvs, nvm, or none.
2. packageManager - Choose your preferred Node package manager. Choose between npm and pnpm.
3. showServiceIncidentList - Show the service health incidents in the account view.
4. showAppsInAppCatalogs - When set it will load all apps in every app catalog from your tenant.
5. createNodeVersionFileDefaultValue - The default value for the new project's setting for creating the Node version file. If checked the default selected value will be YES.
6. nodeVersionManagerFile - The file to be used to store the Node version for the selected Node version manager. Remember that the .node-version file is supported only while using nvs.
7. projectUpgradeOutputMode - Choose the output mode for the project upgrade. You can choose between both, markdown, or code tour.
8. upgradeShellType - Choose the shell type to be used for the upgrade action. You can choose between bash, powershell, or cmd.
9. projectValidateOutputMode - Choose the output mode for the project validation. You can choose between both, markdown, or code tour.

This issue is to create a dedicated page for extension settings listing these and their purpose, default value, and options. let's also link this page in the existing feature pages wherever applicable.