Docs: redo in-page links in Manage extensions, Manage icons, Manage libraries, Manage names, and Manage resources

[activus-d]

This PR fixes the in-page links for some pages in the How to guides section of the documentation by moving single-link block references into the main text. It also adds introductory sentences to some sections to make the docs more user friendly.

This is the #3 in a series of PRs addressing Open Documentation Academy issue #302.

Docs pages covered

• Manage extensions
• Manage icons
• Manage libraries
• Manage names
• Manage resources

Note

Skipped the Manage parts docs as it currently contains TBA (To Be Added) placeholders.