Write changelog in markdown using MyST (#941)

* Install myst-parser and add it to sphinx extension list

MyST (Markedly Structured Text) for writing Sphinx docs in markdown!
Following the installation guide of
[myst-parser](https://github.com/executablebooks/myst-parser) at
https://myst-parser.readthedocs.io/en/v0.13.5/using/intro.html.

* Rename doc/changes.rst to doc/changes.md and convert to markdown

Convert ReST syntax `doc/changes.rst` to MyST syntax `doc/changes.md`.
Mostly changing the GitHub URLs from into the nicer markdown format
Modified template at release-drafter.yml such that:

- A h2 title (##) is used for each Release vX.Y.Z header.
- Alt text for DOI button is more descriptive with version info

* Use h3 html titles for sub-headings
* Let release drafter automatically produce markdown URLs to Pull Requests
* Reorder categories in release note

Order is now "Highlights", "New Features",
"Enhancements", "Deprecations", "Bug Fixes",
"Documentation", and "Maintenance".

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

Author: weiji14

.github/release-drafter.yml

@@ -5,26 +5,29 @@ categories:
     label: 'feature'
   - title: 'Enhancements'
     label: 'enhancement'
-  - title: 'Documentation'
-    label: 'documentation'
+  - title: 'Deprecations'
+    label: 'deprecation'
   - title: 'Bug Fixes'
     label: 'bug'
+  - title: 'Documentation'
+    label: 'documentation'
   - title: 'Maintenance'
     label: 'maintenance'
-  - title: 'Deprecations'
-    label: 'deprecation'
 exclude-labels:
   - 'skip-changelog'
-change-template: '* $TITLE (#$NUMBER)'
+category-template: '### $TITLE'
+change-template: '* $TITLE ([#$NUMBER]($URL))'
 template: |
-  [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3781524.svg)](https://doi.org/10.5281/zenodo.3781524)
+  ## Release v$NEXT_PATCH_VERSION (20YY/MM/DD)
+
+  [![Digital Object Identifier for PyGMT v$NEXT_PATCH_VERSION](https://zenodo.org/badge/DOI/10.5281/zenodo.3781524.svg)](https://doi.org/10.5281/zenodo.3781524)
 
-  ## Highlights
+  ### Highlights
 
   *
 
   $CHANGES
 
-  ## Contributors
+  ### Contributors
 
   $CONTRIBUTORS