Fix format mistake

Fix #604

Only happens for sections with lang != default

Replace trim_{left, right} with trim_{start, end}

trim_{start, end} is introduced in rust 1.30.0 and
trim_{left, right} is deprecated since 1.33.0.

Fix --watch-only to actually rebuild the site

doc: add a Github Action to deploy to Github Pages

Add my blog website to examples file

Footnote is now supported in headers

Add "Hello, Rust!" website to examples file

Make the theme template files render if they are present

* Change the behavior of the template rendering:
    * Check if the template bare name is present
    * Check if the template is part of a theme
    * Fallback to defaults
* Change the behavior of the shortcode rendering:
    * Call the template rendering function
* Prepend `__zola_builtins/` to most of the default elements in `ZOLA_TERA`
* Add a test to verify the presence and content of a `404.html` page
from a theme's template

Closes #582

Closes #576

One more time, Dracula theme, lol

This fixes #569 .

`markdown_to_html` is heavily refactored, header-related things is
handled in a second pass.

* add id to continue reading p tag

Add emphasis, strong and code support in header

this commit contains two refactors:
- extract custom link transformations into a function.
- separate some trivial markup generation.

Closes #554