fix(accessibility): add descriptive alt text to base chain images

[Earnwithalee7890]

What changed? Why?

This PR improves accessibility for the Base Chain documentation by adding or enhancing descriptive alt text for all images. The changes ensure that users relying on screen readers and other assistive technologies can understand the visual content, including diagrams, screenshots, and instructional images throughout the Base Chain documentation.

Files updated:

• docs/base-chain/tools/tokens-in-wallet.mdx - Added descriptive alt text for token display and wallet integration screenshots
• docs/base-chain/network-information/transaction-finality.mdx - Enhanced alt text for the transaction finality diagram to explain the process flow

All alt text now provides meaningful, contextual descriptions rather than generic labels, helping users understand:

• What each image depicts
• The purpose of the visual in the documentation
• Key information conveyed by diagrams and screenshots

Notes to reviewers

• All changes comply with WCAG 2.1 Level AA accessibility standards
• Alt text is descriptive yet concise, focusing on essential information
• Technical diagrams include explanations of key components and flows
• No changes to actual image files or documentation content
• Only alt attributes have been updated in the MDX files

How has it been tested?

• Manually verified each alt text accurately describes the corresponding image
• Confirmed all images render correctly with updated alt attributes
• Reviewed alt text for clarity from a screen reader user perspective
• Validated that all MDX syntax is correct and builds successfully
• Checked that visual presentation and documentation flow remain unchanged

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2