docs(django): document ASGI extra

[SyedAsadRazaDevops]

Description

Documents the optional ASGI extra for Django instrumentation so users running Django through ASGI know to install opentelemetry-instrumentation-django[asgi].

Fixes #1051

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• git diff --check
• python -m py_compile instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/__init__.py

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated: N/A, docs-only change
• Unit tests have been added: N/A, docs-only change
• Documentation has been updated