Fix broken documentation links in developer portal

[soniasingla]

Description

1. Google currently returns /docs/intro/ and /docs/indexer/quickstart/ for the developer portal, but both pages return 404. The current live documentation is under /understand/.

This is likely the first documentation a new developer will encounter when searching for setup/quickstart information.
2. Collections are chain-prefixed, e.g. Polygon__Mainnet__Log rather than Log, but this is not currently documented.

Requested Action

can we set up redirects from the old /docs/intro/ and /docs/indexer/quickstart/ paths to the current /understand/ pages? This is pre-docs revamp, so redirects would prevent new developers from hitting 404s in the meantime.

[johnnymatthews]

Collections are chain-prefixed, e.g. Polygon__Mainnet__Log rather than Log, but this is not currently documented.

This should be a separate issue (made here). This issue will focus on fixing the broken links from Google.