[WIP] Documentation updates for 11.0.0 release

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Making some small improvements before we release

What changes are included in this PR?

• Combine the two CLI pages into a single page and improve formatting
• Move expressions out of DataFrame page and into a new page
• Remove Ballista content
• Fix link to sqlparser source
• Rename "Community" section to "Developer Resources" and moved link to docs.rs there
• Fix broken link to communication page
• Renamed one of the roadmap pages so we don't have two "roadmap" links

This is how the index looks now:

Are there any user-facing changes?

No

[andygrove]

@kmitchener I'm planning on cutting the 11.0.0 release candidate in the next few days, so went ahead and made some small docs improvements here. Apologies if this overlaps a bit with some of your efforts.

[codecov-commenter]

Codecov Report

Merging #3129 (fd54cd7) into master (b1765f7) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3129 +/- ##
==========================================
- Coverage 85.95% 85.94% -0.01% 
==========================================
Files 291 291 Lines 52382 52382 ==========================================
- Hits 45025 45022 -3 - Misses 7357 7360 +3 

Impacted Files	Coverage Δ
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	77.43% <0.00%> (-0.35%)	⬇️

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[andygrove]

Converted to draft because this is in conflict with #3128

[andygrove]

I am going to break this down into smaller PRs.

[kmitchener]

@kmitchener I'm planning on cutting the 11.0.0 release candidate in the next few days, so went ahead and made some small docs improvements here. Apologies if this overlaps a bit with some of your efforts.

ok, I'll hold off on anything more until this latest flurry of changes gets committed. it's looking good!