[Docs] Update next.config.js link to default config

[jflayhart]

It's currently misleading and I chose to freeze to a particular version since pointing to canary every time is likely to break (as is happening prior to this PR).

The link will now go to: https://github.com/vercel/next.js/blob/v10.0.8/packages/next/next-server/server/config-shared.ts#L33

[lfades]

It's not likely we'll change the location of the configs very often, but the configs are more likely to change, so it's okay to point it to canary

Suggested change

	The commented lines are the place where you can put the configs allowed by `next.config.js`, which are defined [here](https://github.com/vercel/next.js/blob/v10.0.8/packages/next/next-server/server/config-shared.ts#L33).
	The commented lines are the place where you can put the configs allowed by `next.config.js`, which are defined [here](https://github.com/vercel/next.js/blob/canary/packages/next/next-server/server/config-shared.ts#L33).

[ijjk]

It might be good to add a comment above this line in the actual code noting we need to update this doc if the location does end up changing in the future.

[timneutkens]

We should document all the options in the docs instead imo.

[jflayhart]

@lfades I committed your suggestion

We should document all the options in the docs instead imo.

@timneutkens if we do that, then we have to update configs in both places: docs and code. Wouldn't it be easier to just maintain a link as a pointer to the code's config object? To @lfades point, the location of the config should rarely change, but properties on the config might (ie experimental stuff); therefore a link to the config is more "future proof".

[lfades]

Thank you!