readthedocs: include all submodules

[proppy]

Signed-off-by: Johan Euphrosine proppy@google.com

Fixes #82

TEST=https://gf180mcu-pdk--83.org.readthedocs.build/en/83/digital/standard_cells/gf180mcu_fd_sc_mcu7t5v0/cells/buf/gf180mcu_fd_sc_mcu7t5v0__buf_20.html

[proppy]

@cbalint13 thanks for the quick review!

[mkkassem]

@proppy the html version still has problems. Even the PDF will be missing items like standard cell libs ... I created my own fork and tried the fix in #83 on my own account for readthedocs.org and still see issues. See attached images. Top not working correctly and bottom is working when I did a local build.