Add comprehensive documentation for supported assets and timeframes

[Copilot]

This PR adds comprehensive documentation for all supported assets and timeframes in the PocketOption API, addressing the community request for clarity on what trading instruments are available.

What's Added

New Documentation Page

• Created /docs/assets-timeframes.html with complete listings of all supported trading instruments
• Professional styling that matches the existing documentation design
• Organized content by asset categories for easy navigation

Comprehensive Asset Coverage (121 total assets)

• Major Forex Pairs (7): EURUSD, GBPUSD, USDJPY, etc.
• OTC Forex Pairs (25): Extended trading hours currency pairs
• Commodities (14): Gold, Silver, Oil, Natural Gas, Platinum, Palladium
• Cryptocurrencies (10): Bitcoin, Ethereum, Dash, Polkadot, Chainlink
• Stock Indices (24): S&P 500, NASDAQ, Dow Jones, Nikkei 225, DAX 30, etc.
• US Stocks (36): Apple, Microsoft, Tesla, Amazon, Netflix, etc.
• Additional Assets (5): Alternative stock symbol formats

Complete Timeframe Documentation (14 timeframes)

• From ultra-high frequency (1 second) to long-term (1 month)
• Both TimeFrame enum values and raw seconds provided
• Clear use case descriptions for each timeframe (scalping, swing trading, etc.)

Enhanced User Experience

• Navigation menu updated across all documentation pages
• Practical usage examples with code samples
• Important notes about asset availability and trading hours
• Copy-to-clipboard functionality for code examples
• Responsive grid layout for easy browsing

Code Examples

The documentation includes practical examples showing how to use the asset symbols:

# Getting candles for different asset types
forex_candles = await client.get_candles("GBPUSD_otc", TimeFrame.H1, 24)
crypto_candles = await client.get_candles("BTCUSD", TimeFrame.M15, 96)
stock_candles = await client.get_candles("#AAPL", TimeFrame.D1, 30)

This documentation eliminates confusion about which assets are supported and provides developers with the exact symbols needed for API calls.

Fixes #38.

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