Add support for nightly allocator_api-feature

[seijikun]

I want to write an application using the ESP32S3's LCD peripheral.
This peripheral works by allocating a framebuffer that contains the entire display content (For me: 800 x 480 pxl) and then streaming that out to the LCD using DMA.
For obvious reasons, this allocation has to be done in PSRAM, not in DRAM.

I haven't found a way to do that except for using Vec::with_capacity_in - which requires the nightly only allocator_api.

This PR implements support for this feature.
I also added some easy getting started documentation on the docs frontpage while I was at it.

[seijikun]

Okay, extremely weird. The GitHub PR preview shows

let large_buffer: Vec<u8, _> = Vec::with_capacity(1048576, &PSRAM_ALLOCATOR);

even though the code in that line actually is:

let large_buffer: Vec<u8, _> = Vec::with_capacity_in(1048576, &PSRAM_ALLOCATOR);

So you can ignore that - only a GitHub bug.

[jessebraham]

Thank you for the PR, I wasn't aware of this feature!

I think you're just running into some browser caching issues or something, as the line you mentioned is displayed correctly for me!