Taichi: [Doc] Improve documentation

[yuanming-hu]

Concisely describe the proposed feature
There are quite a few things to be completed in the documentation

• Sparse computation
• Debugging tools
• Differentiable programming
• Internal designs

[archibate]

Also want a page clearly declare API usage in the manner like doxygen/pydoc, not just explain by showing examples.

See kOS's documention on Vector for example, listing all methods of this Vector class, and all equipped with a description and type/access info. Some complex methods are also equipped with a example usage below like vectorangle().
They not only have very friendly user guides, but also full documention of Structures, Functions, Language, etc. Seperately describe its API into much smaller granularity.

Comparing to our Vector documention:
You see we simply put R, S = ti.polar_decompose(A, ti.f32) and no further desc about what R, S, A are provided. User who doesn't familiar such single-char-denotions might ask, what is R and S? May even reverse them?

And not to mention we even don't have a constructor description like ti.Vector(n, m, dt=xxx)...
TBH I had a hard time figuring out the usage of x = ti.Vector([1, 2])...

[archibate]

Most searched doc:

• [x] gui 18 searches
• [x] place 12 searches
• [x] vector 12 searches
• [x] print 10 searches
• [ ] ti.static 9 searches
• [x] ti.var 9 searches
• [ ] debug 8 searches
• [x] root 8 searches
• [x] dense 7 searches
• [x] layout 7 searches

Let's make these complete first.

[lvjiahui]

Also eager to read the doc about sparse computation in taichi.