This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.
If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:
```toml
[markup]
defaultMarkdownHandler="blackfriday"
```
Fixes#5963Fixes#1778Fixes#6355
And use that to calculate number of workers, if set, else fall back to number of logical CPUs.
Also tweak the relevant related settings to match the new setup.
Also remove the setting of `runtime.GOMAXPROCS` as this has been the default behaviour since Go 1.5.
Fixes#5814
This allows for "cache per Hugo project", making `hugo --gc` work as expected, even if you have several Hugo projects running on the same PC.
See #5439
