generate doc with pandoc in docker

[laktak]

@joelim-work here is the script in its own PR.

You can update the pandoc version but 2.11 was the closest to 2.5 that was mentioned in the existing docs.

Also I would not actually check doc.txt and lf.1 into git as they should be generated during the build step.

[joelim-work]

Thanks, the script looks good to me.

I was thinking about introducing this as a CI check similar to go generate (now obsolete), but I'm not 100% sure about this since it will require contributors to either setup Docker, or keep manually fixing the documentation mistakes until the build passes.

Regarding the autogenerated doc.txt and lf.1 files, this was also discussed in #1474, but @gokcehan decided to keep them as there doesn't seem to be a good solution. The main problem is that using go install to build from source won't work if doc.txt doesn't exist.

[gokcehan]

I was just using the pandoc installed in my ubuntu 20.04 system, which is quite outdated. I think it is okay to use a newer version or even the latest version. Alternatively, we can also try to use the pandoc version in the latest ubuntu or latest ubuntu-lts releases.