Speed up documentation build

[matt-graham]

Resolves (some of) #1513

• Remove linkcheck step
• Removes -f force option to sphinx-apidoc and -E fresh env default argument to sphinx-build
• Removes sphinx.ext.viewcode extension
• Stops last updated date text being added to footers to minimize diff noise
• Changes maximum resource file size before switching to just including a placeholder page linking to file to download rather than rendering table to 32KiB rather than 1MiB - this was a quicker alternative to suggestion in Speed up documentation generation #1513 to use an online viewer. This would still be nice to have but as a lot of resource files are still XLSX can be delayed till we have finished with converting them to CSV.

Times for tox -e docs steps before and after changes (in both cases from a clean state):

Step	Time (master) / s	Time (this PR) / s
Setup	8.89	8.06
sphinx-apidoc	0.31	0.29
tlo_methods_rst.py	7.51	7.51
tlo_data_sources.py	0.34	0.37
tlo_contributors.py	0.06	0.06
tlo_publications.py	0.38	0.42
tlo_resources.py	8.17	1.18
hsi_events.py (RST)	6.95	7.01
hsi_events.py (CSV)	7.75	7.00
tlo_parameters.py	4.95	5.10
sphinx-build	216.90	39.63
linkcheck	720.06	(removed)
Total	982.26	76.63

Biggest gain is removing linkcheck step but sphinx-build step is also significantly reduced, mainly because of previously some of the large resource files taking a while to render.

[tamuri]

Thanks - much better!