Cleanup documentation warnings

[jlucas91]

Generating the documentation produces a number of warnings, which could be disconcerting or confusing for first time contributors. This PR cleans up those warnings.

It also:

• Adds "What's new in V1" to the documentation navigation.
• Cleans up some language in the contributors guide.

Existing warnings:

             - whatsnew_v1.md
INFO    -  Doc file 'motivation.md' contains an absolute link '/guides/versioning/', it was left as is. Did you mean
           'guides/versioning.md'?
INFO    -  Doc file 'motivation.md' contains an absolute link '/guides/response/#returning-querysets', it was left as is. Did you mean
           'guides/response/index.md#returning-querysets'?
INFO    -  Doc file 'motivation.md' contains an absolute link '/guides/response/django-pydantic/', it was left as is. Did you mean
           'guides/response/django-pydantic.md'?
INFO    -  Doc file 'motivation.md' contains an absolute link '/guides/authentication/', it was left as is. Did you mean
           'guides/authentication.md'?
INFO    -  Doc file 'guides/input/form-params.md' contains an unrecognized relative link '../body/#declare-it-as-a-parameter', it was
           left as is. Did you mean 'body.md#declare-it-as-a-parameter'?
INFO    -  Doc file 'guides/input/form-params.md' contains an unrecognized relative link '../body/#request-body-path-query-parameters',
           it was left as is. Did you mean 'body.md#request-body-path-query-parameters'?
INFO    -  Doc file 'guides/response/index.md' contains an unrecognized relative link '../async-support#using-orm', it was left as is.
           Did you mean '../async-support.md#using-orm'?
INFO    -  Doc file 'guides/response/config-pydantic.md' contains an unrecognized relative link
           '../django-pydantic-create-schema/#create_schema', it was left as is. Did you mean
           'django-pydantic-create-schema.md#create_schema'?
INFO    -  Doc file 'guides/response/django-pydantic-create-schema.md' contains an unrecognized relative link
           '../django-pydantic/#modelschema', it was left as is. Did you mean 'django-pydantic.md#modelschema'?
INFO    -  Doc file 'proposals/index.md' contains an unrecognized relative link 'cbv', it was left as is. Did you mean 'cbv.md'?

[jlucas91]

@vitalik just pinging you here in case you missed this. Hopefully is pretty straightforward to merge?

[vitalik]

Thank you