convert navigation2 tutorials to rst format for github pages

[mlherd]

Manually converted the following tutorials to rst format.

• Navigation2 on a Real Turtlebot 3
• Navigation2 with Turtlebot 3 in Gazebo

Copied the doc images to sphinx_doc directory.

Changes in PR #1393 may affect this PR. (doc tree and links etc)

[SteveMacenski]

Just some rewording, comment inline and we can discuss

[mhpanah]

Personally I prefer not to use "I" , "you", "we" in technical documents. This document is written in a second person narrative with too many "you"s. I will approve, if that's okay with you.

[mlherd]

Personally I prefer not to use "I" , "you", "we" in technical documents. This document is written in a second person narrative with too many "you"s. I will approve, if that's okay with you.

These are just tutorials, so I picked this language to give the user the feeling that "we" go through each step together. I didn't want this to sound like a boring product manual that no one ever reads :)

[codecov]

Codecov Report

Merging #1428 into master will decrease coverage by 1.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1428      +/-   ##
==========================================
- Coverage   40.28%   38.59%   -1.69%     
==========================================
  Files         229      229              
  Lines       11765    11765              
  Branches     5074     5074              
==========================================
- Hits         4739     4541     -198     
- Misses       3627     3872     +245     
+ Partials     3399     3352      -47

Flag	Coverage Δ
#project	38.59% <ø> (-1.69%)	⬇️

Impacted Files	Coverage Δ
...er/dwb_core/include/dwb_core/dwb_local_planner.hpp	0% <0%> (-100%)	⬇️
nav2_core/include/nav2_core/controller.hpp	50% <0%> (-50%)	⬇️
...controller/dwb_core/include/dwb_core/publisher.hpp	0% <0%> (-50%)	⬇️
...oller/include/nav2_controller/progress_checker.hpp	50% <0%> (-50%)	⬇️
...er/dwb_critics/include/dwb_critics/oscillation.hpp	33.33% <0%> (-33.34%)	⬇️
...ler/dwb_critics/include/dwb_critics/goal_align.hpp	66.66% <0%> (-33.34%)	⬇️
...ler/dwb_critics/include/dwb_critics/path_align.hpp	66.66% <0%> (-33.34%)	⬇️
...d/include/nav2_costmap_2d/costmap_2d_publisher.hpp	69.23% <0%> (-30.77%)	⬇️
nav2_recoveries/src/recovery_server.cpp	15.06% <0%> (-23.29%)	⬇️
nav2_waypoint_follower/src/waypoint_follower.cpp	11.86% <0%> (-21.19%)	⬇️
... and 33 more

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[SteveMacenski]

I think the only thing left from my review is the turtlebot links