Improve grid snaping docs, functions and names

[joamatab]

• snap path extrusion and transition, and add docs for snapping
• movement of paths and polygons snaps movements to grid to avoid 1nm gaps
• add tutorial on grid_snapping issues and fixes
• rename enable_ports_off_grid to enable_ports_offgrid to be consistent, deprecate the old one

@yaugenst
@simbilod
@mdecea
@nikosavola
@HelgeGehring
@tvt173

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (e94d5fa) 72.55% compared to head (498f0bd) 72.59%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
gdsfactory/config.py	75.00%	4 Missing ⚠️
gdsfactory/cell.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2446      +/-   ##
==========================================
+ Coverage   72.55%   72.59%   +0.04%     
==========================================
  Files         366      366              
  Lines       23289    23306      +17     
  Branches     3789     3789              
==========================================
+ Hits        16898    16920      +22     
  Misses       5264     5264              
+ Partials     1127     1122       -5     

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