Closes #366

* Let toc is visable through Page & Section variables in templates

* Removed the current toc variable from page & section

* Bump pulldown_cmark to version 0.6.0

* Rename headers to headings

* fix the issue of generating the search index for multiple language

* updat docs for generating the search index for multiple language

* fix failed tests

* add tests for the search index of multiple language

* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site

To ensure we will get the right `content`
directory.
Fix #629

Closes #607

Only happens for sections with lang != default

Closes #576

Closes #485

Close #446
Close #260
Close #478
Close #284
Close #480

Fix #122

Fix #205

Push summary handling into Markdown parsing, identifying the presence
of one by giving its length in the rendered markup.

Hopefully a better fix for #376.

This reverts commit c7156a84f0.