feat(cli): support import of .md files

Signed-off-by: Phil Kuang <pkuang@factset.com>

.changeset/eleven-coats-refuse.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Support importing `.md` files in build loader

docs/local-dev/cli-build-system.md

@@ -505,6 +505,7 @@ of all supported file extensions:
 | `.jpg`      | URL Path        | Image                                                                         |
 | `.png`      | URL Path        | Image                                                                         |
 | `.svg`      | URL Path        | Image                                                                         |
+| `.md`       | URL Path        | Markdown File                                                                 |
 | `.icon.svg` | React Component | SVG converted into a [MUI SvgIcon](https://mui.com/components/icons/#svgicon) |
 
 ## Jest Configuration

packages/cli/asset-types/asset-types.d.ts

@@ -40,6 +40,11 @@ declare module '*.jpeg' {
   export default src;
 }
 
+declare module '*.md' {
+  const src: string;
+  export default src;
+}
+
 declare module '*.png' {
   const src: string;
   export default src;

packages/cli/src/lib/builder/config.ts

@@ -132,6 +132,7 @@ export async function makeRollupConfigs(
               /\.woff$/,
               /\.woff2$/,
               /\.ttf$/,
+              /\.md$/,
             ],
           }),
           json(),