Readthedocs 1.4: Two missing components on the Ubuntu install script

[JL102]

Hello,

There are two problems with the install script provided on the readthedocs documentation:

1. python3.6-dev, as included in the install script, is not available on current versions of Ubuntu. See https://stackoverflow.com/questions/43621584/why-cant-i-install-python3-6-dev-on-ubuntu16-04#44735144
2. The package autoconf is required for installing verilator in the end of the script, but is not included in the list of items to install, nor does it come with Ubuntu 20.04. It should probably be added to the items to be installed.

Minor issues of course, but just FYI. (Side note: Thank you very much for having such good and clear documentation. I greatly appreciate it, it's made setting up chipyard very easy.)

[alonamid]

I believe ubuntu 18.04LTS and 20.04LTS have pre-installed later versions of Python, so not have python3.6 should not impact Chipyard (python3.6 is required mostly for 16.04LTS).

I find it quite surprising the autoconf isn't preinstalled with Ububtu, but we will happily accept a PR to add autoconf to the ubuntu requirements list in the docs

[abejgonzalez]

Closing since this should be resolved by #874