Update indexers.md

[karelxfi]

Please add SQD to [Tooling, Indexers]. We have been supporting Avalanche for some time: https://docs.subsquid.io/subsquid-network/reference/evm-networks/. Thank you.

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Why this should be merged

How this works

How these changes were tested

To prevent any errors while building

• [] run vale /docs/file/path on all changed .md files to ensure all grammar rules pass
• [] run markdownlint /docs/file/path on all changed .md files to ensure all linting rules pass
• [] complete the above two checks and all additional rules outlined in style-checker-notes.md to ensure all checks pass

If a document was removed/deleted

• [] the path to that doc must be redirected to a valid URL via the _redirects file
• [] _redirects were manually verified with the cloudflare preview link
• [] sidebars.json reflects all changes made to file path

If a document was moved

• [] all files that were moved from their current directory to a new path have had their paths redirected via the _redirects file
• [] _redirects were manually verified with the cloudflare preview link
• [] sidebars.json reflects all changes made to file path