feat(build): include code from external sources

[vgvoleg]

Closes #2207

Adds {% include-code %}: a document references a file in an external source, and the build embeds
its content instead of a hand-written copy.

code-sources:
  python-sdk:
    type: git
    repo: ydb-platform/ydb-python-sdk
    ref: main
    path: examples

{% include-code [Connecting to a database](python-sdk:static-credentials/example.py#auth-static) %}

Three source types: git (a repository on a hosting service), http (plain files), local (a
directory on disk). Fragments are addressed by named regions (#region name or [START name]),
line ranges are supported but warned about.

Notes:

• Nothing is cloned and no git binary is used: the ref is resolved over git smart HTTP, then only
  the referenced files are downloaded. One file out of a large monorepo costs kilobytes.
• The directive is lowered to an ordinary fenced block before the rest of markdown processing, so
  HTML, PDF, single-page, md2md, search and llms.txt need no changes.
• Links are pinned to the resolved commit, with line numbers derived from the region.
• A missing region fails the build — that is the point: it turns silent doc rot into a build error.

Two additions outside the feature directory: Run.addScope, so downloaded files are read through the
existing path sandbox rather than around it, and an export of findFencedCodeBlockRanges, so a
directive shown as a code example is left alone by the same rule {% include %} uses.

Not covered: private repositories, a lock file, incremental/watch tracking of external files.
User documentation will follow as a separate PR to diplodoc-platform/docs.

[martyanovandrey]

Thanks! We're discussing the approach internally with the author, will follow up here