Create a documentation directory and updating the serial subsystem docs

[Courtney3141]

This pull request aims to provide an example of a preferred style of documentation. Firstly, it adds a docs directory to the repo, which contains developer targeted documentation for using each sDDF subsystem. Secondly, it restricts the README.md documentation in the serial example to more specifically provide details of the example, leaving general subsystem informations for docs/serial.md.

Once a convention for such documentation is agreed upon, we should aim to update the remaining sDDF subsystems.

[erichchan999]

I'm halfway through on a block doc aswell, will put that into this PR when im closer to finishing

[Ivan-Velickovic]

I'd like to discuss with others in the team the role of this documentation vs the design doc before reviewing. I don't have any strong opinions on whether we should have everything in one place or not, but I do want to understand what belongs in the design doc and what belongs in these more technical docs.

[Ivan-Velickovic]

I haven't looked closely yet but one good addition would be listing exactly what drivers we have and any setup they need.

Right now we have stuff like this in the Makefile:

# NOTES
#  Generates eth_driver.elf
#  Expects System Description File to set eth_regs to the address of
#  the registers
#  Expects libsddf_util_debug.a to be in LIBS

Other than the last line about LIBS, we really should not be documenting things in the Makefiles themselves.

So for serial we should have something like:

We support the following drivers:
* PL0111
* etc

Each driver expects to have the `uart_base` variable patched in the System Description File (SDF) like so:
<map mr="uart" vaddr="0x5_000_000" perms="rw" cached="false" setvar_vaddr="uart_base" />

[Ivan-Velickovic]

This should be showing example output, we should always include example output of what the system should look like when you boot it up so people know if they've done the right thing or not.

[Ivan-Velickovic]

There used to be a section called "Running/using" https://github.com/au-ts/sddf/blob/95492dcc32abf8f8b4fc34297ba5c26fb181d891/examples/serial/README.md#runningusing.

We should have that.

[Ivan-Velickovic]

@erichchan999 can you put this in a separate PR, it'll be hard to address feedback for two people's work in the same PR. It's fine for your PR to just be based of Courtney's branch,

[erichchan999]

sure, i'll undo those commits

[Ivan-Velickovic]

Suggested change

	separation of concerns which communicate via shared data structures and microkit channels.
	separation of concerns which communicate via shared data structures and asynchronous notifications (Microkit channels).

[Ivan-Velickovic]

Suggested change

	In order to transmit and receive via the uart device, clients of the system must interface with
	In order to transmit and receive via the UART device, clients of the system must interface with

[Ivan-Velickovic]

Before addressing any feedback I would suggest rebasing since there might be some conflicting changes in the serial example since the PR got made.