feat(explorer): add dependabot

[glpecile]

Dependabot is a common practice to keep dependencies up-to-date.

This PR adds a dependabot, which checks for dependency updates, and makes a new PR updating them, if needed.

This PR limits dependabot PR creations to up to 1 per week, and up to 5 open PRs at the same time.

[glpecile]

@uri-99 the image below basically shows that the config is correct and will start publishing updates once merged.

[Oppen]

Converted to draft due to not being a priority for mainnet

[JuArce]

Stale