doc: Fix minor things on `communication` doc

[kavirajk]

Which issue does this PR close?

Closes NA.

Rationale for this change

Having correct links on communication page is generally helpful and particularly for new contributors

What changes are included in this PR?

Changes:

1. Good first issue link was broken & was pointing wrongly to datafusion repository
2. Update repo links (datafusion -> ballista)
3. Remove spec reference that was pointing to datafusion specification.

Are there any user-facing changes?

NO

NO

[kavirajk]

@andygrove any idea CI (Docker stage) is not passing. Looks totally unrelated to the PR.

[3/9] COPY target/release/ballista-scheduler /root/ballista-scheduler
#6 ERROR: failed to calculate checksum of ref abc794c5-012e-49ab-b658-c7ad62c56cb6::00d8a19ciim46e09f5gy4g04f: "/target/release/ballista-scheduler": not found

#7 [5/9] RUN chmod a+x /root/ballista-scheduler &&     chmod a+x /root/ballista-executor
#7 CACHED

#8 [6/9] RUN mkdir -p /data &&     wget -q https://d37ci6vzurychx.cloudfront.net/trip-data/yellow_tripdata_2022-01.parquet -P /data/
#8 CACHED

#9 [7/9] COPY ballista/scheduler/ui/build /var/www/html
#9 ERROR: failed to calculate checksum of ref abc794c5-012e-49ab-b658-c7ad62c56cb6::00d8a19ciim[46](https://github.com/apache/datafusion-ballista/actions/runs/8775461142/job/24159808966?pr=1006#step:5:47)e09f5gy4g04f: "/ballista/scheduler/ui/build": not found

#10 [4/9] COPY target/release/ballista-executor /root/ballista-executor
#10 ERROR: failed to calculate checksum of ref abc794c5-012e-49ab-b658-c7ad62c56cb6::00d8a19ciim46e09f5gy4g04f: "/target/release/ballista-executor": not found

Looks like copying anything from /target/release/ and ui/build is failing.

Also this may be the one blocking other PRs as well?