Local developement docs are outdated

[AyanSinhaMahapatra]

It seems we have recently moved to a docker based local development instead of a virtualenv one with:

• https://github.com/aboutcode-org/scancode.io/pull/2145/changes

The documentation has not been updated and this is outdated:

• https://scancodeio.readthedocs.io/en/latest/installation.html#local-development
• http://github.com/aboutcode-org/scancode.io/blame/main/docs/installation.rst#local-development

[nik-767]

Implemented the documentation update to reflect the current Docker-based local development workflow. Looking forward to any feedback. Thank you!