OTel spec: link to local OTLP, and fix README link

[chalin]

• Followup to Update opentelemetry-specification version to 1.22.0 #2856
• Closes Adjust to move of OTLP specs into opentelemetry-proto #2642
• Ensures that external links to the OTLP spec become a local path inside OTel spec pages
• Also includes a workaround for the _serve:hugo script, avoiding the error message Error: command error: unknown command "-DFE" for "hugo server" when stopping the server

Here are the affected files:

$ (cd public && git diff | grep ^diff)                                                               
diff --git a/docs/specs/otel/protocol/exporter/index.html b/docs/specs/otel/protocol/exporter/index.html
diff --git a/docs/specs/otel/protocol/file-exporter/index.html b/docs/specs/otel/protocol/file-exporter/index.html
diff --git a/docs/specs/otel/protocol/index.xml b/docs/specs/otel/protocol/index.xml
diff --git a/docs/specs/otel/protocol/otlp/index.html b/docs/specs/otel/protocol/otlp/index.html

And an example of the change:

$ (cd public && git diff -I '<meta' docs/specs/otel/protocol/otlp/index.html)
diff --git a/docs/specs/otel/protocol/otlp/index.html b/docs/specs/otel/protocol/otlp/index.html
index bed2713b3..9838d69a1 100644
--- a/docs/specs/otel/protocol/otlp/index.html
+++ b/docs/specs/otel/protocol/otlp/index.html
@@ -1536,7 +1536,7 @@
                
        </header>
        <p>This page has moved to
-<a href="https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md" target="_blank" rel="noopener" class="external-link">github.com/open-telemetry/opentelemetry-proto/docs/specification.md</a>.</p>
+<a href="/docs/specs/otlp/">/docs/specs/otlp/</a>.</p>