docs: Establish VISION for "One Layer, All Storage"

[Xuanwo]

Which issue does this PR close?

Result of https://github.com/apache/opendal/discussions/5301

Rationale for this change

Establish VISION for "One Layer, All Storage"

What changes are included in this PR?

Update docs that refer to vision.

Are there any user-facing changes?

Not in the short term, but it will affect the entire future.

[Xuanwo]

I also invite all @apache/opendal-committers to join the review.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #5309 will degrade performances by 15.89%

_{Comparing add-vision (271b697) with main (de198cd)}

Summary

⚡ 8 improvements ❌ 4 regressions ✅ 61 untouched benchmarks

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	add-vision	Change
❌	buffer 256 KiB * 32 chunk	183.6 ns	212.8 ns	-13.71%
❌	buffer 256 KiB * 4 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 4.00 MiB * 32 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 4.00 MiB * 4 chunk	154.4 ns	183.6 ns	-15.89%
⚡	256 KiB * 1000k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 1000k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 100k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 100k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 10k chunk	212.8 ns	183.6 ns	+15.89%
⚡	256 KiB * 10k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 1k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 1k truncate	245.8 ns	216.7 ns	+13.46%

[Xuanwo]

Thank you all for the review, especially @tisonkun for the detailed comments and @xxchan for optimizing the expression. I'm going to merge this now.

Let's build "One Layer" for "All Storage"!