Remove docfx (#4036)

DocFx v3 is no longer available and not intended to be used externally.

The feature it provided that we don't have from other tooling is
cross-file anchor reference check, see
open-telemetry/semantic-conventions#1009 for
the details. It seems there is no popular (or even not popular)
replacement for it.

DocFx v2 only check anchors within the same file which markdownlint does
as well.

Related:
open-telemetry/semantic-conventions#1008

Closes #4026.

.github/workflows/checks.yaml

@@ -68,17 +68,3 @@ jobs:
 
     - name: run misspell
       run: make misspell
-
-  docfx:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: 6.0.x
-
-    - name: install docfx
-      run: dotnet tool update -g docfx --version "3.0.0-*" --add-source https://docfx.pkgs.visualstudio.com/docfx/_packaging/docs-public-packages/nuget/v3/index.json
-
-    - name: run docfx
-      run: docfx build --dry-run