Add Accept header to curl commands in traffic docs

[tomastauber-da]

ported from canton-network/splice#4298

[Jatinp26]

@tomastauber-da did the debug on variable validation on the current changes, it seems to fail in MDX Page currently being in the commit with the changes. Basically, the committed synchronizer-traffic.mdx no longer matches what the generator produces.

Could you please update the forked branch based on current content on main and then redit your content and make a new commit @tomastauber-da ?

Below is the specific error debug log block from the failed job outta the above check.

> python3 scripts/validate_network_variable_tabs.py
Network variable tabs are stale. Run `npm run generate:network-variable-tabs` and commit the rendered MDX changes.
Updated 1 page(s).
 M docs-main/global-synchronizer/deployment/synchronizer-traffic.mdx
Error: Process completed with exit code 1
```.