various documentation updates

[kmitchener]

Which issue does this PR close?

Closes #1352 Closes #3001 Closes #2866 You can also close #2558 and close #1813 though they're not directly related to this PR

Rationale for this change

What changes are included in this PR?

• removes the python docs per issue #2866 (there's a related PR in the new python repo to bring the docs over there https://github.com/apache/arrow-datafusion-python/pull/23
• fix some links and typos and do consolidation of duplicate material
• replace some .rst files with .md in
• fix docker build for datafusion-cli and update docs related to that
• improve left nav link naming for clarity (under Specification)
• consolidated the documentation for the CLI into one page per issue #1352
• fix the CSV schema inference in datafusion-cli docs per #3001
• update README.md to point to new location of DataFusion Python project
• update README.md with link to slides from Andrew Lamb and Daniel Heres

Are there any user-facing changes?

The docs are slightly better.

[codecov-commenter]

Codecov Report

Merging #3005 (210012e) into master (9a5f17e) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #3005    +/-   ##
========================================
  Coverage   85.84%   85.85%            
========================================
  Files         283      289     +6     
  Lines       51658    51890   +232     
========================================
+ Hits        44347    44550   +203     
- Misses       7311     7340    +29     

Impacted Files	Coverage Δ
datafusion/physical-expr/src/aggregate/build_in.rs	89.92% <0.00%> (-0.26%)	:arrow_down:
datafusion/proto/src/to_proto.rs	53.03% <0.00%> (-0.19%)	:arrow_down:
datafusion/core/tests/sql/mod.rs	98.10% <0.00%> (-0.17%)	:arrow_down:
datafusion/proto/src/from_proto.rs	35.53% <0.00%> (-0.05%)	:arrow_down:
datafusion/physical-expr/src/aggregate/stddev.rs	96.03% <0.00%> (-0.04%)	:arrow_down:
...afusion/physical-expr/src/aggregate/correlation.rs	97.95% <0.00%> (-0.02%)	:arrow_down:
datafusion/physical-expr/src/aggregate/variance.rs	98.30% <0.00%> (-0.02%)	:arrow_down:
...tafusion/physical-expr/src/aggregate/covariance.rs	98.70% <0.00%> (-0.01%)	:arrow_down:
datafusion/core/tests/sql/timestamp.rs	100.00% <0.00%> (ø)
datafusion/core/tests/parquet_pruning.rs	99.43% <0.00%> (ø)
... and 22 more

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[kmitchener]

Updated per review. I'm not done with this. Still working on organizing the content and creating new content.

[andygrove]

@kmitchener What do you think about breaking some of these changes out into smaller PRs? For example, the formatting changes to the roadmap could be reviewed as a standalone change. This might make it easier to review the more substantive changes here.

[kmitchener]

@andygrove sure, yes, I can do that. I got a little exploratory and just kept making changes, unsure how they would work out. I'll submit separate PR for the simple cleanups and see how I can break this up further for easier review. I've been away from this for a few days, maybe fresh eyes will help.

[kmitchener]

closing this PR as it was exploratory and too big and all it's changes are going in other PRs