make API docs link more prominant

[ZZiigguurraatt]

We have a small link to the API documentation tucked away at the very BOTTOM of the home page (https://docs.lightning.engineering/).

docs.lightning.engineering/README.md

Line 66 in 0ba7c3c

Additional external resources include our [Developer Slack](https://lightning.engineering/slack.html), [Github organization](https://github.com/lightninglabs), and [API documentation, including LND, Loop, Pool, Faraday & Taproot Assets](https://lightning.engineering/api-docs/).

I think the API documentation is a very big deal, why hide it down there? Can we put it as an item on the left sidebar so that it shows up on every documentation page that you are browsing? Driving people to the API documentation can help fill people's gaps in the regular documentation and even if you understand the regular documentation, you really want a quick way to always jump there. Remembering to type docs.lightning.engineering into your browser and then click a prominent API link is something people are going to remember to quickly do, but remembering or bookmarking https://lightning.engineering/api-docs/api/lnd/index.html is not. Sure, we can go to https://api.lightning.community/ as a shortcut, but most people don't know that.