Skip anchor checking for URL with prefix in config

[TjeuKayim]

Fixes #805

zola check

The anchors of external URLs will not be checked if it starts with one of the configured prefixes.
So false positives can be put on the ignore list, like https://github.com/rust-lang/rust/blob/c772948b687488a087356cb91432425662e034b9/src/librustc_back/target/mod.rs#L194-L214: Anchor `#L194-L214 and https://caniuse.com/#feat=css-filters.

# Link Checker: skip anchor checking for external URLs that start with these prefixes
link_checker_skip_anchor_prefixes = [
    "https://caniuse.com/",
]

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

[Keats]

Just a small change, everything looks great otherwise.

[Keats]

Great, thanks!