docs: update agent development documentation

[pingsutw]

What changes were proposed in this pull request?

• Update the existing agent example
• Add a sensor example
• Add a screenshot to show how to run BigQuery and Databricks tasks on your local machine

doc links:

• https://flyte--5130.org.readthedocs.build/en/5130/flyte_agents/developing_agents.html
• https://flyte--5130.org.readthedocs.build/en/5130/flyte_agents/testing_agents_locally.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.02%. Comparing base (c81133b) to head (c85631d).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5130      +/-   ##
==========================================
- Coverage   59.03%   59.02%   -0.01%     
==========================================
  Files         645      645              
  Lines       55726    55726              
==========================================
- Hits        32896    32895       -1     
- Misses      20233    20234       +1     
  Partials     2597     2597              

Flag	Coverage Δ
unittests	59.02% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[neverett]

Left some suggestions, thanks for updating these docs!