[MINOR][DOCS] Add link to Hadoop docs

[martin-kokos]

What changes were proposed in this pull request?

Add link to Writable in Hadoop docs.

Why are the changes needed?

The python card of of the blue talks about Writables with no explanation what it is and the meaning of the whole paragraph is lost. While the Scala card links to Hadoop docs, the python card did not.

Does this PR introduce any user-facing change?

Help docs readability.

How was this patch tested?

Link is tested accessible.

[itholic]

Can we create corresponding JIRA ticket here, and add a JIRA number to the title??
e.g. [SPARK-12345][DOCS] Add link to Hadoop docs
or we can just add [MINOR] tag without creating ticket to JIRA, when the change is small enough.
e.g. [MINOR][DOCS] Add link to Hadoop docs

Btw, you can find more information for contributing Apache Spark here.

[itholic]

Also please check https://github.com/apache/spark/pull/39941/checks?check_run_id=11190702189 out to enable GitHub Actions in your forked repository

[itholic]

Anyway, seems fine to me when the test passed.

[itholic]

While we're here, maybe can we fix other miscellaneous things together if you have some time?

e.g.
Writables -> Writable
IntWritables -> IntWritable
Texts -> Text
InputFormats -> InputFormat
...

They're not actually very big deal, though.

[martin-kokos]

Fixed by c9c3880