Myst-parser

✨ NEW: Add `html_admonition` extension

: By adding `"html_admonition"` to `myst_enable_extensions`, you can enable parsing of `<div class="admonition">` HTML blocks to sphinx admonitions.
: This is helpful when you care about viewing the "source" Markdown, such as in Jupyter Notebooks.
: For example:
html
<div class="admonition note" name="html-admonition">
<p class="title">This is the **title**</p>
This is the *content*
</div>

: See [the optional syntax guide](docs/syntax/optional.md) for further information.

👌 IMPROVE: Footnotes

: If the label is an integer, then it will always use this integer for the rendered label (i.e. they are manually numbered).
: Add `myst_footnote_transition` configuration, to turn on/off transition line.
: Add `footnotes` class to transition `<hr>` in HTML.
: See [the typography guide](docs/syntax/typography.md) for further information.

👌 IMPROVE: `substitution` extension logic

: Parse inline substitutions without block rules, unless the substitution starts with a directive.

🐛 FIX: Render front-matter as `field_list`

: To improve use by sphinx extensions).

👌 IMPROVE: Code quality

: Add isort and mypy type checking to code base.

(thanks to contributors <gh-user:akhmerov>, <gh-user:tfiers>)

👌 Directives can now be used for inline substitutions, e.g.

md
---
substitutions:
key: |
{image} img/fun-fish.png
:alt: fishy
:height: 20px

---

An inline image: {{ key }}

This release makes some major updates to the optional syntaxes.
For full details see [Optional MyST Syntaxes](docs/syntax/optional.md).

🗑 Deprecations

`myst_enable_extensions = ["dollarmath", ...]` now replaces and deprecates individual enable configuration variables: `admonition_enable` -> `"colon_fence"`, `figure_enable` -> `"colon_fence"`, `dmath_enable` -> `"dollarmath"`, `amsmath` -> `"colon_fence"`, `deflist_enable` -> `"deflist"`, `html_img_enable` -> `"html_image"`.

The `colon_fence` extension (replacing `admonition_enable`) now works exactly the same as normal ` ` code fences, but using `:::` delimiters. This is helpful for directives that contain Markdown text, for example:

md
:::{admonition} The title
:class: note

This note contains *Markdown*
:::


✨ New

The `smartquotes` extension will automatically convert standard quotations to their opening/closing variants:

- `'single quotes'`: ‘single quotes’
- `"double quotes"`: “double quotes”

The `linkify` extension will automatically identify “bare” web URLs, like `www.example.com`, and add hyperlinks; www.example.com.
This extension requires that [linkify-it-py](https://github.com/tsutsu3/linkify-it-py) is installed.

The `replacements` extension will automatically convert some common typographic texts, such as `+-` -> `±`.

The `substitution` extension allows you to specify "substitution definitions" in either the `conf.py` (as `myst_substitutions`) and/or individual file's front-matter (front-matter takes precedence), which will then replace substitution references. For example:

md
---
substitutions:
key1: definition
---
{{ key1 }}


The substitutions are assessed as [jinja2 expressions](http://jinja.palletsprojects.com/) and includes the [Sphinx Environment](inv:sphinx#extdev/envapi) as `env`, so you can do powerful thinks like:


{{ [key1, env.docname] | join('/') }}


The `figure-md` directive has been added (replacing `enable_figure`), which parses a "Markdown friendly" figure (used with the `colon_fence` extension):

md
:::{figure-md} fig-target
:class: myclass

<img src="img/fun-fish.png" alt="fishy" class="bg-primary mb-1" width="200px">

This is a caption in **Markdown**
:::


👌 Improvements

Using the `html_image` extension, HTML images are now processed for both blocks and (now) inline.

So you can correctly do, for example:

md
I’m an inline image: <img src="img/fun-fish.png" height="20px">

| table column |
| ----------------------------------------- |
| <img src="img/fun-fish.png" width="20px"> |

🐛 FIX: allow dates to be parsed in frontmatter.
: This fixes a bug that would raise errors at parse time if non-string date objects were in front-matter YAML. See <gh-pr:253>

✨ NEW: Auto-generate heading anchors.
: This utilises `markdown-it-py`'s `anchors-plugin`, to generate unique anchor "slugs" for each header (up to a certain level),
and allows them to be referenced *via* a relative path, e.g. `[](./file.mdheader-anchor)`, or in the same document, e.g. `[](header-anchor)`.

Slugs are generated in the GitHub style ([see here](https://github.com/Flet/github-slugger)); lower-case text, removing punctuation, replacing spaces with `-`, enforce uniqueness *via* suffix enumeration `-1`.

It is enabled in your `conf.py` *via* `myst_heading_anchors = 2` (sets maximum heading level).

See [the documentation here](docs/syntax/optional.mdauto-generated-header-anchors).

🐛 FIX: doc reference resolution for singlehtml/latex.
: These reference resolutions are passed to the "missing-reference" event, and require the `node["refdoc"]` attribute to be available, which was missing for `[text](./path/to/file.md)` type references.