Add contribution chapter
Port the change log
Use el as default domain

Makes directives and roles shorter, and improves the info output,
because Sphinx doesn't add the role name to description nodes anymore
Add manual license
Port the credits
Fix cross-reference to FDL
Port syntax checker index
Improve rendering of syntax checkers
Port @xref links

Also resolve them properly for HTML output
Use a custom role for info cross references
Remove the old texinfo manual and related files
Re-license Sphinx extensions

There is no reason to put these under GPL.
Split extensions out of conf.py
Turn issue references in the changelog into links
Add current info builds
Update the changelog
Re-work the front page
Raise extensions headline
Shorten README
Update info manual