Add Panel layout docs and enhancements [doc-build]

[philippjfr]

No description provided.

[codecov-io]

Codecov Report

Merging #347 into master will increase coverage by 0.11%.
The diff coverage is 90.86%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   86.94%   87.05%   +0.11%     
==========================================
  Files          88       88              
  Lines        7583     7741     +158     
==========================================
+ Hits         6593     6739     +146     
- Misses        990     1002      +12

Impacted Files	Coverage Δ
panel/tests/widgets/test_slider.py	100% <ø> (ø)
panel/tests/widgets/test_player.py	100% <ø> (ø)
panel/tests/widgets/test_button.py	100% <ø> (ø)
panel/tests/widgets/test_input.py	100% <ø> (ø)
panel/tests/pane/test_equation.py	100% <ø> (ø)
panel/tests/widgets/test_select.py	100% <ø> (ø)
panel/tests/pane/test_vega.py	96.61% <ø> (ø)
panel/tests/pane/test_markup.py	100% <ø> (ø)
panel/tests/pane/test_plotly.py	97.46% <ø> (ø)
panel/tests/pane/test_image.py	100% <ø> (ø)
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc0b7d8...0ada9e6. Read the comment docs.