Add note in docs about excluding workflow file from dependency update tools

I use [Renovate](https://www.mend.io/renovate/) on my repositories which tends to look at the cargo-dist generated workflow, find that it's running builds on `ubuntu-20.04` and "helpfully" upgrades it to `ubuntu-22.04` which then breaks the plan step of the next release.

It might be of some value to document in the docs how to disable this for some of the popular dependency update solutions: [Renovate can do this](https://github.com/msfjarvis/rucksack/commit/fc6469128ac8195830dcc2e9516efc5ce58d0134), but it seems Dependabot itself doesn't support such a feature yet (https://github.com/dependabot/dependabot-core/issues/4364).

I can send in a PR if this is desirable and someone lets me know what page this belongs to.

Review the documentation structure to identify the appropriate page for guidance about cargo-dist's generated workflow. Document how Renovate can exclude the workflow and note that Dependabot does not currently support the same feature; verify the wording against the linked Renovate example and Dependabot issue.

Written by the indexing model from the issue text.