Restructure style guide page and add detailed style guide guidelines.

[deshmukhpranali]

Tasks

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1. Restructure the 'Contributing and Style Guide' page into pages and subpages
2. Create 'Contribution Guidelines' page under the 'Contributing and Style Guide' page.
3. Create 'Style Guide' page under the 'Contributing and Style Guide' page.
4. Under the 'Contribution Guidelines' create pages for 'Creating Issues', 'Creating Pull Requests', 'Hedera Improvement Proposal (HIP)' and 'Submit Demo Applications'.
5. Under the 'Style Guide' create pages for 'Understanding different types of documentation', 'Use of HBAR', 'Use of web2 and web3', 'Language and grammar', 'Formatting' and 'Punctuation'.