[demo] services-table merge deleted an alias and forgot to add one

[chalin]

• Context [demo] move services table into service index page #3833
• When a page is deleted from the website, we generally need to add an alias to avoid 404s.
• Also, in the case of 3833, an already defined alias was lost, and needs to be restored:

  diff --git a/_redirects b/_redirects
  index 3df07902e..5a5885f01 100644
  --- a/_redirects
  +++ b/_redirects
  @@ -176,7 +177,6 @@ docs/specs/semconv/general/metrics-general /docs/specs/semconv/general/metrics/
   /docs/reference/specification/schemas/overview /docs/specs/otel/schemas/
   /docs/demo/demo_screenshots         /docs/demo/screenshots/
   /docs/reference/specification/sdk-configuration /docs/specs/otel/configuration/sdk-configuration/
  -/docs/demo/service_table            /docs/demo/service-table/
   /docs/demo/services/shippingservice /docs/demo/services/shipping/
   /project-status                     /status/
   /releases                           /status/