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 [Question] interested in helping with documentation #2866 (there's a related PR in the new python repo to bring the docs over there moved python docs from DataFusion project datafusion-python#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 documentations for datafusion-cli can be consolidated a bit more #1352
• fix the CSV schema inference in datafusion-cli docs per User guide is incorrect regarding using CLI to register CSV files using schema inference #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%)	⬇️
datafusion/proto/src/to_proto.rs	53.03% <0.00%> (-0.19%)	⬇️
datafusion/core/tests/sql/mod.rs	98.10% <0.00%> (-0.17%)	⬇️
datafusion/proto/src/from_proto.rs	35.53% <0.00%> (-0.05%)	⬇️
datafusion/physical-expr/src/aggregate/stddev.rs	96.03% <0.00%> (-0.04%)	⬇️
...afusion/physical-expr/src/aggregate/correlation.rs	97.95% <0.00%> (-0.02%)	⬇️
datafusion/physical-expr/src/aggregate/variance.rs	98.30% <0.00%> (-0.02%)	⬇️
...tafusion/physical-expr/src/aggregate/covariance.rs	98.70% <0.00%> (-0.01%)	⬇️
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