Make Algolia configurable via GitHub vars (not secrets)

[ghost]

What does this PR fix?

Currently Algolia config is stored in repository secrets, although it is not secret at all:

That is why we should use variables instead - it will make configuration inspection easier.

Checklist

• My changes follow the Casper docs style guidelines.
• All technical procedures have been tested.

[ghost]

I created new variables, according to the current Algolia configuration:

The bottom ones (with _SITE_) are the configuration for the navbar search.

TODO: Remove relevant secrets, once this PR gets merged.

[bradjohnl]

As discussed, approving this PR. We will test using the correct index for navbar and docs separately as it's something that requires playing with vars only

[ghost]

1. Backported to docs-new in casper-network/docs-new@7d58a74.
2. Unnecessary secrets removed from docs and docs-new.

[ghost]

For the record, docs variables are updated to the new values provided by @bradjohnl:

docs-new as well:

On both environments navbar search works as expected, e.g.