chore: migrate non-blog content to YAML frontmatter

Convert all content files to use YAML (---) instead of TOML (+++)
for consistent parsing:

- content/_index.md (homepage)
- content/about.md, content/collab.md
- content/blog/_index.md
- content/projects/_index.md + 14 project cards

Enables unified gray_matter YAML parser across all content.

content/projects/digga.md

@@ -1,9 +1,6 @@
-+++
-title = "digga"
-description = "A Nix flake utility library for managing nixos, hm and devshells"
-weight = 3
-
-
-[extra]
-link_to = "https://digga.divnix.com/"
-+++
+---
+title: digga
+description: "A Nix flake utility library for managing nixos, hm and devshells"
+weight: 3
+link_to: "https://digga.divnix.com/"
+---