Blobs migration update (Azure#16295)

* Synced TOC with page structure

* Expanded AAD migration in migrations guide.

Added v11 info on how AAD auth was handled.
Changed v12 section to discuss Azure.Identity package, linking to their
samples.
Our v12 sample uses DefaultAzureCredential as a quick sample instead of
ClientSecredCredential, as that is the starting place recommended in the
Identity SDK readme.

* Modified SAS auth migration guide for Blob Storage

Auth section now shows how to authenticate with an existing SAS.
SAS generation was moved to its own section.

* Blob migration guide connection string touchup

Side-by-side samples now more closely mirror each other.
Added v12 shortcut where you can skip directly to a BlobClient while
using connection string to authenticate.

* Blob migrations guide added shared key section

Added shared key auth side-by-side examples as an addition to connection
string examples. This allows migrating customers to see necessary
migration examples if they want the ability to rotate their shared key
in long lived clients.

* Blob migration guide refactor client comparison

Explained difference between legacy object model and modern client
model.
Fixed 11v12 class equivalent table.
Added suggestion of data lake SDK and hierarchical namespace for those
whose workloads involve directory manipulation.
Explained BlobClient.

* blobs migration guide upload/download

Updated uploads and downloads to have more closely mirrored examples.
Introduced download mechanisms exclusive to v12

* blob migration guide flat listing

Updated flat listings to contain side-by-side examples for both lazy
enumerables and manual page iteration for each of the SDKs.

* blob migration guide list by hierarchy

Added side by side samples for hierarchical listing

* blob migration guide removed empty section

* blob migration guide stated scope modified

Guide originally stated it is for migrating from v11 to v12.
Now states it is for migrating from legacy versions in general to v12.
Notes that the guide was written with v11 in mind, and that migration
samples are created using v11 for reference.

* Addressed smaller comments

* v12 samples now labeled as snippets

* Initial sample cs file

* update snippets

* build errors

Co-authored-by: jschrepp-MSFT <41338290+jschrepp-MSFT@users.noreply.github.com>

Author: jaschrep-msft