Fix menu links for "welcome" anchors.

This fixes the broken "How to Contribute" and "Spread the Word" links in the menu.

In the `index.html`, `{{ welcome_content|markdownify }}` generates name
attributes for header tags like "my-title", whereas `{{ post.content }}`
generates name attributes like "my_title".

This fixes the links for only the welcome menu items so that the anchors
in the menu match the different name format.

I couldn't figure out how to change the name format, so at least this fixes the
links, even if it isn't consistent.