Updates to targets JSON and related stuff for the website generator

Summary of changes

This PR makes a few changes to targets.json5 and the related code to support the new target & driver website generator. Changes made:

Rename features.json5 -> drivers.json5. This is a better name as the umbrella term "driver" can now be used to refer to peripherals, features, and/or components. At the end of the day, when we talk about DEVICE_SPI or COMPONENT_QSPIF, we are ultimately talking about a driver for SPI or a QSPIF device, so I think this makes sense.
Make some grammar fixes in devices.json5
Mark targets that I consider to be "MCU family targets" in targets.json5 as such. These targets (as explained in the targets website) are targets which are the parents of an entire family of boards based around substantially similar MCUs. It isn't a perfectly exact science, but I feel like these groupings make the targets listing a fair bit easier to understand.
Fix bug in mbed_tools python package where getting the attributes of a target would modify fields of the top level target (Target). This would make targets appear to have peripheral drivers and components that they didn't actually have (!). Added a new regression test to look for this issue.

Impact of changes

This is really only to support the website generator and shouldn't contain any user visible changes.

Migration actions required

Documentation

This is primarily a docs related PR!

Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

mbed-ce / mbed-os

Updates to targets JSON and related stuff for the website generator #265