feat: add convert-excel-to-md, convert-pdf-to-md, and convert-word-to-md skills (#2294)

* feat: add convert-excel-to-md, convert-pdf-to-md, and convert-word-to-md skills

Add three new agent skills that convert common document formats to Markdown
using bundled Python scripts powered by MarkItDown:

- convert-excel-to-md: Converts .xlsx workbooks to Markdown with per-sheet
  tables and embedded image extraction via a bundled Python script.
- convert-pdf-to-md: Converts .pdf documents to Markdown with text/table
  extraction and embedded image extraction via PyMuPDF.
- convert-word-to-md: Converts .docx documents to Markdown with proper
  image extraction replacing MarkItDown's base64 placeholders.

Each skill includes:
- SKILL.md with detailed usage instructions, output structure docs, and
  a troubleshooting table
- scripts/ with the conversion Python script and requirements.txt
- references/setup.md with environment setup instructions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: remove Markdown image syntax from convert-word-to-md SKILL.md

The CI valid-refs linter flagged the literal Markdown image syntax
containing a data URI as an invalid file reference. Replaced it with
a plain text description of the placeholder format.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: update pip install command to use scripts/requirements.txt for setup

* fix: clarify installation instructions for requirements-file in setup documentation

* fix: correct indentation in image extraction function

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* feat: add comprehensive documentation and setup instructions for convert-to-md skills

* fix: add trailing slashes to skill paths in plugin.json

* fix: reorder skills in plugin.json for consistency

* fix: update plugin.json and README.md for clarity and consistency

* feat: add convert-to-md plugin and update related documentation

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix: enhance conversion skills to handle mixed file types and improve error handling

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

plugins/convert-to-md/.github/plugin/plugin.json

@@ -0,0 +1,24 @@
+{
+  "name": "convert-to-md",
+  "description": "A collection of Copilot skills that convert common document formats into Markdown so their contents can be accurately analyzed, summarized, searched, or extracted from. Just tell Copilot what you need — the right skill is invoked automatically and the conversion happens behind the scenes.",
+  "version": "1.0.1",
+  "keywords": [
+    "skills",
+    "configuration",
+    "copilot",
+    "convert-word-to-md",
+    "convert-excel-to-md",
+    "convert-pdf-to-md"
+  ],
+  "author": {
+    "name": "Willie Yao",
+    "url": "https://github.com/1YaoWei0"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "skills": [
+    "./skills/convert-excel-to-md/",
+    "./skills/convert-pdf-to-md/",
+    "./skills/convert-word-to-md/"
+  ]
+}