adjust-spec: process img paths of schemas index page

[chalin]

• Contributes to:
  • Integrate the OTel specification into the website #906
  • [editorial] Ensure every section has a README opentelemetry-specification#2558
• Ensure img paths get processed properly for the schemas section once the following lands and the spec is updated in this repo: [editorial] Ensure schemas section has a README, and other changes opentelemetry-specification#3046
• This PR has no effect on the currently generated site files (but it will, once the spec PR 3046 is integrated), so it is safe to merge before (or whether) PR 3046 is merged.

/cc @yurishkuro @bogdandrutu

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Screenshot

Here's a screenshot of one of the file format pages once PR 3046 is integrated, along with this PR:

[cartermp]

👍🏻 -- will defer to you on merge order