Re-organize Tahoe-LAFS manual's table of contents

[sajith]

The manual is hard-to-navigate in its current shape. As a first step of making it easier to consume, we could re-organize the table of contents to several sections:

• General introduction to Tahoe-LAFS
• Content meant for beginner users (obtaining, running, commands, etc.)
• Content meant for advanced users (tor, i2p, sftp frontend, logging, etc.)
• Content meant for contributors (architecture, development practices, REST API, release checklist, documentation, etc.)
• Community-related content (donations, governance, etc)
• Proposals
• Content that is of historical relevance (old configuration files, pyOpenSSL, building on Windows/OSX, etc)