[BEAM-7389] Generate notebook for filter

[davidcavazos]

First PR introducing the script to generate notebooks from docs using the Filter page as an example.

R: @aaltay [script/code]
R: @rosetn [writing/formatting]

Staged: http://apache-beam-website-pull-requests.storage.googleapis.com/9561/documentation/transforms/python/elementwise/filter/index.html
Notebook: https://colab.research.google.com/github/davidcavazos/beam/blob/notebook-filter/examples/notebooks/documentation/transforms/python/element-wise/filter-py.ipynb

This is using the newly created md2ipynb package to generate the notebook files from Markdown. It's importing the license and setup instructions which are not present in the source Markdown docs, but needed in the notebook. It also adds links to back to the docs.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[davidcavazos]

Staged: http://apache-beam-website-pull-requests.storage.googleapis.com/9561/documentation/transforms/python/elementwise/filter/index.html
Notebook: https://colab.research.google.com/github/davidcavazos/beam/blob/notebook-filter/examples/notebooks/documentation/transforms/python/element-wise/filter-py.ipynb