Support GH links to canonical src in lang repo

[chalin]

Contributes to #542

/cc @carlosalberto @mtwo @shelbyspees

As mentioned in the file, this is a Docsy file override. Here is the diff:

$ diff {themes/docsy/,}layouts/partials/page-meta-links.html
0a1
> {{/* otel-docsy file override */ -}}
16a18
> {{ $gh_repo_path = replace $gh_repo_path ($.Param "path_base_for_github_subdir") "" -}}

[chalin]

Exceptionally, I've also submitted a commit matching the changes in open-telemetry/opentelemetry-java#3501 so that you can validate the change. (I can either pull out that commit before merging, or leave it in -- I don't think that it should cause problems for the website_docs PR that will be eventually created from the OTel Java repo.)

Preview: https://deploy-preview-687--opentelemetry.netlify.app/docs/java/ -- from there you can also check the subpages.

Note that Edit this page, for example, now links to https://github.com/open-telemetry/opentelemetry-java/edit/main/website_docs/_index.md.

[chalin]

Note in passing how this split of the docs (as introduced by #472) makes it more challenging to preview changes. #661 will make it possible to preview changes without having to commit changes in two repos.

[chalin]

When creating an issue using the Create documentation issue link, the issue title is set to the page title by default. We'll likely want to tweak that (by, say, adding a prefix like Website:), but I suggest that we address this in a followup issue & PR.

[chalin]

Rebased after merging OTel Java change.