feat: add getDocsHandler() for mounting docs page manually

[sjorobekov]

Allow manual mounting of swagger page, which will allow skipping expresskit's beforeAuth middlewares calls

How to use

const { registerRoutes, getDocsHandler } = createOpenApiRegistry({ skipMount: true, /* … */ });
const app = new ExpressKit(nodekit, registerRoutes(routes, nodekit));
app.express.use('/api/docs', getDocsHandler());