Update Discord API vanity invite

[SinisterRectus]

Regrettably, the vanity invite for the Discord API server has been changed from discord-api to api. This PR updates the link in the official docs. To any readers, please spread the word and update your links in any third-party resources.

[night]

hmmmm .. it might be time to actually have the docs link our official developers server instead, and instead link API as a support resource on the libraries page (or have per-library support links)

[SinisterRectus]

Neither the Intro page nor the classic API docs link to any support server. There is a link to "Discord API" on the Community Resources page, and on this repo's readme, while "Discord Developers" is linked on top of all SDK and Dispatch pages. If you want to fix that up, go ahead, but my goal here is to just make the current links accurate.

We could add per-library links to the community page, although not every library has its own support server.

[advaith1]

Looks like there was a link to DAPI in the homepage but it was moved to community resources in 3f48e71.

imo:

• update readme to link to devs server
• add link to devs server in homepage
• fix link to API server in community resources
• add per-lib links to community resources (to its support server if available, or to its dapi channel)

[SinisterRectus]

The vanity URL has been reverted back to discord-api. Closing this as the changes are no longer accurate.