Merge master into 3.4.x (better mdoc, Apache license) (#1605)

* Fix broken mdoc (#1600)

* Fix broken mdoc

* Added test-chisel-docs to ci workflow

* Move Chisel3 to SPDX license conventions (#1604)

Change source and other relevant files to use SPDX license
LICENSE file moved from src/ to ./
Changed license file to refer to this per recommendation
using_spdx_license_list_short_identifiers

WARNING: Tests fail with as of yet undiagnosed error
```
[error] Failed: Total 691, Failed 19, Errors 0, Passed 672, Ignored 15
[error] Failed tests:
[error] 	chiselTests.QueueSpec
[error] 	examples.VendingMachineGeneratorSpec
[error] 	chiselTests.HarnessSpec
[error] 	chiselTests.ConnectSpec
[error] 	chiselTests.aop.SelectSpec
[error] 	chiselTests.PopCountSpec
[error] 	chiselTests.CloneModuleSpec
[error] (Test / test) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 379 s (06:19), completed Sep 30, 2020 12:38:17 AM
sbt:chisel3>
```

* Move more docs (#1601)

* Fix broken mdoc

* Added test-chisel-docs to ci workflow

* Add requirement on build-treadle job

* Added forgotton colon

* Forgot cd into chisel3 dir

* moved three docs into explanations

* Updated reference

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>

Co-authored-by: Adam Izraelevitz <azidar@gmail.com>
Co-authored-by: Chick Markley <chick@qrhino.com>