Add GLFM Specification Guide to docs

- Describes plan for implementation

doc/.vale/gitlab/spelling-exceptions.txt

@@ -90,6 +90,7 @@ Caddy
 callstack
 callstacks
 Camo
+canonicalization
 canonicalized
 captcha
 CentOS
@@ -517,6 +518,7 @@ reinitialize
 reinitializing
 relicensing
 remediations
+renderers
 replicables
 repmgr
 repmgrd
@@ -646,6 +648,8 @@ subtrees
 sudo
 supercookie
 supercookies
+superset
+supersets
 supertype
 supertypes
 swappiness

doc/development/gitlab_flavored_markdown/index.md

+---
+stage: Create
+group: Editor
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Markdown developer documentation **(FREE)**
+
+This page contains the MVC for the developer documentation for GitLab Flavored Markdown.
+For the user documentation about Markdown in GitLab, refer to
+[GitLab Flavored Markdown](../../user/markdown.md).
+
+## GitLab Flavored Markdown specification guide
+
+The [specification guide](specification_guide/index.md) includes:
+
+- [Terms and definitions](specification_guide/index.md#terms-and-definitions).
+- [Parsing and rendering](specification_guide/index.md#parsing-and-rendering).
+- [Goals](specification_guide/index.md#goals).
+- [Implementation](specification_guide/index.md#implementation) of the spec.