Transfer issues from main repository

[alerque]

For history & context having site related issues here seems like a good thing. I migrated all the issues that were directly related to the site. Links to migrated issues auto-redirect to their new numbers on this repo.

PR's cannot be migrated, but in the event the history is relevant:

• Cleanup layout and typesetting of website sile#291
• Add development history visualization post and cleanup GH-Pages code sile#289
• Update manual to current version number sile#375

Also mentions in these issues, but as long as the related source is in the sile repo it seemed like leaving the related issue was best.

• Example source + PDF sile#12
• Provide a compelling show-case document sile#343