[IMP] Inventory: Check availability changed to reserve button

[erjer-odoo]

What this PR does and why it's needed

Updates inventory reservation documentation to reflect the UI change where the Check Availability button was renamed to Reserve. Also refreshes surrounding content and screenshots in the affected pages to match current field labels and workflows.

Changes

Manually reservation method

• Replaced Check Availability button references with Reserve throughout the workflow
• Small updates for line length and fixes according to our style guide

Before scheduled date reservation method

• Replaced Check Availability button reference with Reserve
• Small updates for line length and fixes according to our style guide

Batch transfers

• Replaced Check Availability button reference with Reserve in the operations tab example

• Small updates for line length and fixes according to our style guide

• Screenshots were updated throughout to reflect the current UI.

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This saas-19.4 PR can be FWP up to master.

[robodoo]

[erjer-odoo]

Hi @theRealThagomizer -- I've got a lot of small updates to the same UI label throughout several docs. Because the changes were fairly minor and largely related to that one setting, I've assigned points accordingly.

This is ready for review when you get a moment. Thank you!