DOC-648 flytesnacks edits needed for Neptune and W&B flytekit plugins documentation

[neverett]

In this PR:

• Update neptune and W&B refs so the flytekit plugin docs for these plugins link to the flytekit documentation, not the flytesnacks examples
• Add sections to left nav to match sections in page
• Copyedits

This is one of three PRs we need to merge in the following order to get the neptune and wandb flytekit plugin docs onto the docs site:

1. flytesnacks PR (this one) to fix refs in neptune and wandb plugin docs
2. flyte PR to update monodocs dependencies
3. flytekit PR to add new flytekit plugin docs for neptune and wandb plugins (after re-running monodocs build)